XMPP Working Group P. Saint-Andre (ed.)
Internet-Draft Jabber Software Foundation
Expires: May 20, 2004 November 20, 2003
Extensible Messaging and Presence Protocol (XMPP): Core
draft-ietf-xmpp-core-20
Status of this Memo
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This Internet-Draft will expire on May 20, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo defines the core features of the Extensible Messaging and
Presence Protocol (XMPP), a protocol for streaming Extensible Markup
Language (XML) elements in order to exchange structured information
in close to real time between any two network endpoints. While XMPP
provides a generalized, extensible framework for exchanging XML data,
it is used mainly for the purpose of building instant messaging and
presence applications that meet the requirements of RFC 2779.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Generalized Architecture . . . . . . . . . . . . . . . . . . . 4
3. Addressing Scheme . . . . . . . . . . . . . . . . . . . . . . 6
4. XML Streams . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Use of TLS . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Use of SASL . . . . . . . . . . . . . . . . . . . . . . . . . 26
7. Resource Binding . . . . . . . . . . . . . . . . . . . . . . . 36
8. Server Dialback . . . . . . . . . . . . . . . . . . . . . . . 39
9. XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . . 45
10. Server Rules for Handling XML Stanzas . . . . . . . . . . . . 54
11. XML Usage within XMPP . . . . . . . . . . . . . . . . . . . . 57
12. Core Compliance Requirements . . . . . . . . . . . . . . . . . 59
13. Internationalization Considerations . . . . . . . . . . . . . 61
14. Security Considerations . . . . . . . . . . . . . . . . . . . 61
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 65
Normative References . . . . . . . . . . . . . . . . . . . . . 68
Informative References . . . . . . . . . . . . . . . . . . . . 70
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 71
A. Nodeprep . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
B. Resourceprep . . . . . . . . . . . . . . . . . . . . . . . . . 73
C. XML Schemas . . . . . . . . . . . . . . . . . . . . . . . . . 75
D. Differences Between Core Jabber Protocol and XMPP . . . . . . 82
E. Revision History . . . . . . . . . . . . . . . . . . . . . . . 84
Intellectual Property and Copyright Statements . . . . . . . . 92
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1. Introduction
1.1 Overview
The Extensible Messaging and Presence Protocol (XMPP) is an open XML
[XML] protocol for near-real-time messaging, presence, and
request-response services. The basic syntax and semantics were
developed originally within the Jabber open-source community, mainly
in 1999. In 2002, the XMPP WG was chartered with developing an
adaptation of the Jabber protocol that would be suitable as an IETF
instant messaging (IM) and presence technology. As a result of work
by the XMPP WG, the current memo defines the core features of XMPP;
Extensible Messaging and Presence Protocol (XMPP): Instant Messaging
and Presence [XMPP-IM] defines the extensions required to provide the
instant messaging and presence functionality defined in RFC 2779
[IMP-REQS].
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 [TERMS].
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.
1.5 Contributors
Most of the core aspects of the Extensible Messaging and Presence
Protocol were developed originally within the Jabber open-source
community in 1999. This community was founded by Jeremie Miller, who
released source code for the initial version of the jabberd server in
January 1999. Major early contributors to the base protocol also
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included Ryan Eatmon, Peter Millard, Thomas Muldowney, and Dave
Smith. Work by the XMPP Working Group has concentrated especially on
security and internationalization; in these areas, protocols for the
use of TLS and SASL were originally contributed by Rob Norris, and
stringprep profiles were originally contributed by Joe Hildebrand.
The error code syntax was suggested by Lisa Dusseault.
1.6 Acknowledgements
Thanks are due to a number of individuals in addition to the
contributors listed. Although it is difficult to provide a complete
list, the following individuals were particularly helpful in defining
the protocols or in commenting on the specifications in this memo:
Thomas Charron, Richard Dobson, Sam Hartman, Schuyler Heath, Jonathan
Hogg, Craig Kaes, Jacek Konieczny, Alexey Melnikov, Keith Minkler,
Julian Missig, Pete Resnick, Marshall Rose, Alexey Shchepin,
Jean-Louis Seguineau, Iain Shigeoka, and David Waite. Thanks also to
members of the XMPP Working Group and the IETF community for comments
and feedback provided throughout the life of this memo.
2. Generalized Architecture
2.1 Overview
Although XMPP is not wedded to any specific network architecture, to
date it usually has been implemented via a typical client-server
architecture, wherein a client utilizing XMPP accesses a server over
a TCP [TCP] 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
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o FC1 -- A client on a foreign messaging network
2.2 Server
A server acts as an intelligent abstraction layer for XMPP
communications. Its primary responsibilities are:
o to manage connections from or sessions for other entities, in the
form of XML streams (Section 4) to and from authorized clients,
servers, and other entities
o to route appropriately-addressed XML stanzas (Section 9) among
such entities over XML streams
Most XMPP-compliant servers also assume responsibility for the
storage of data that is used by clients (e.g., contact lists for
users of XMPP-based instant messaging and presence applications); in
this case, the XML data is processed directly by the server itself on
behalf of the client and is not routed to another entity.
2.3 Client
Most clients connect directly to a server over a [TCP] 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 [HTTP] 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 differentiated
by the resource identifier of an XMPP address (e.g., <node@domain/
home> vs. <node@domain/work>) as defined under Addressing Scheme
(Section 3). The RECOMMENDED port for connections between a client
and a server is 5222, as registered with the IANA (see Port Numbers
(Section 15.9)).
2.4 Gateway
A gateway is a special-purpose server-side service whose primary
function is to translate XMPP into the protocol used by a foreign
(non-XMPP) messaging system, as well as to translate the return data
back into XMPP. Examples are gateways to Internet Relay Chat (IRC),
Short Message Service (SMS), SIMPLE, 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.
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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 SHOULD occur over XML streams that are bound to
[TCP] sockets. The RECOMMENDED port for connections between servers
is 5269, as registered with the IANA (see Port Numbers (Section
15.9)).
3. Addressing Scheme
3.1 Overview
An entity is anything that can be considered a network endpoint
(i.e., an ID on the network) and that can communicate using XMPP.
All such entities are uniquely addressable in a form that is
consistent with RFC 2396 [URI]. For historical reasons, the address
of an XMPP entity is called a Jabber Identifier or JID. A valid JID
contains a set of ordered elements formed of a domain identifier,
node identifier, and resource identifier. The syntax is defined below
using Augmented Backus-Naur Form as defined in RFC 2234 [ABNF], where
the "node", "domain", and "resource" identifiers are as specified in
the following sections:
[ node "@" ] domain [ "/" resource ]
Each allowable portion of a JID (node identifier, domain identifier,
and resource identifier) MUST NOT be more than 1023 bytes in length,
resulting in a maximum total size (including the '@' and '/'
separators) of 3071 bytes.
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@host/
resource>. However, node types other than clients are possible; for
example, a specific chat room offered by a multi-user chat service
could be addressed as <room@service> (where "room" is the name of the
chat room and "service" is the hostname of the multi-user chat
service) and a specific occupant of such a room could be addressed as
<room@service/nick> (where "nick" is the occupant's room nickname).
Many other JID types are possible (e.g., <domain/resource> could be a
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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 be a Fully Qualified Domain Name.
A domain identifier MUST be an "internationalized domain name" as
defined in [IDNA], to which the Nameprep [NAMEPREP] profile of
stringprep [STRINGPREP] can be applied without failing. Before
comparing two domain identifiers, a server MUST (and a client SHOULD)
first apply the Nameprep profile to the labels (as defined in [IDNA])
that make up each identifier.
3.3 Node Identifier
The node identifier is an optional secondary identifier placed before
the domain identifier and separated from the latter by the '@'
character. It usually represents the entity requesting and using
network access provided by the server or gateway (i.e., a client),
although it can also represent other kinds of entities (e.g., a chat
room associated with a multi-user chat service). The entity
represented by a node identifier is addressed within the context of a
specific domain; within instant messaging and presence applications
of XMPP this address is called a "bare JID" and is of the form
<node@domain>.
A node identifier MUST be formatted such that the Nodeprep (Appendix
A) profile of [STRINGPREP] can be applied to it without failing.
Before comparing two node identifiers, a server MUST (and a client
SHOULD) first apply the Nodeprep profile to each identifier.
3.4 Resource Identifier
The resource identifier is an optional tertiary identifier placed
after the domain identifier and separated from the latter by the '/'
character. A resource identifier may modify either a <node@domain>
or mere <domain> address. It usually represents a specific session,
connection (e.g., a device or location), or object (e.g., a
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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 Resource Binding (Section 7) (although it may be generated
by a server on behalf of a client). An entity MAY maintain multiple
resources simultaneously, with each resource differentiated by a
distinct resource identifier.
A resource identifier MUST be formatted such that the Resourceprep
(Appendix B) profile of [STRINGPREP] can be applied to it without
failing. Before comparing two resource identifiers, a server MUST
(and a client SHOULD) first apply the Resourceprep profile to each
identifier.
3.5 Determination of Addresses
After SASL negotiation (Section 6) and, if appropriate, Resource
Binding (Section 7), the receiving entity for a stream MUST determine
the initiating entity's JID.
For server-to-server communications, the initiating entity's JID
SHOULD be the authorization identity, derived from the authentication
identity as defined by the Simple Authentication and Security Layer
(SASL) specification [SASL] if no authorization identity was
specified during SASL negotiation (Section 6).
For client-to-server communications, the "bare JID" (<node@domain>)
SHOULD be the authorization identity, derived from the authentication
identity as defined in [SASL] if no authorization identity was
specified during SASL negotiation (Section 6); the resource
identifier portion of the "full JID" (<node@domain/resource>) SHOULD
be the resource identifier negotiated by the client and server during
Resource Binding (Section 7).
The receiving entity MUST ensure that the resulting JID (including
node identifier, domain identifier, resource identifier, and
separator characters) conforms to the rules and formats defined
earlier in this section; to meet this restriction, the receiving
entity may need to replace the JID sent by the initiating entity with
the canonicalized JID as determined by the receiving entity.
4. XML Streams
4.1 Overview
Two fundamental concepts make possible the rapid, asynchronous
exchange of relatively small payloads of structured information
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between presence-aware entities: XML streams and XML stanzas. These
terms are 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 [XML]).
The start of any XML stanza is denoted unambiguously by the
element start tag at depth=1 of the XML stream (e.g., <presence>),
and the end of any XML stanza is denoted unambiguously by the
corresponding close tag at depth=1 (e.g., </presence>). An XML
stanza MAY contain child elements (with accompanying attributes,
elements, and CDATA) as necessary in order to convey the desired
information. The only defined XML stanzas are <message/>,
<presence/>, and <iq/> as defined under XML Stanzas (Section 9);
an XML element sent for the purpose of Transport Layer Security
(TLS) negotiation (Section 5), Simple Authentication and Security
Layer (SASL) negotiation (Section 6), or server dialback (Section
8) is not considered to be an XML stanza.
Consider the example of a client's session with a server. In order
to connect to a server, a client MUST initiate an XML stream by
sending an opening <stream> tag to the server, optionally preceded by
a text declaration specifying the XML version and the character
encoding supported (see Inclusion of Text Declaration (Section 11.4);
see also Character Encoding (Section 11.5)). Subject to local
policies and service provisioning, the server SHOULD then reply with
a second XML stream back to the client, again optionally preceded by
a text declaration. Once the client has completed SASL negotiation
(Section 6), the client MAY send an unbounded number of XML stanzas
over the stream to any recipient on the network. When the client
desires to close the stream, it simply sends a closing </stream> tag
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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 in a simplistic
fashion 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 a hostname serviced by the receiving entity. There
SHOULD be no 'to' attribute set in the XML stream header by which
the receiving entity replies to the initiating entity; however, if
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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 a hostname serviced by the receiving entity that is
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 xml:lang -- An 'xml:lang' attribute (as defined in Section 2.12 of
[XML]) SHOULD be included by the initiating entity on the header
for the initial stream to specify the default language of any
human-readable XML character data it sends over that stream. If
the attribute is included, the receiving entity SHOULD remember
that value as the default for both the initial stream and the
response stream; if the attribute is not included, the receiving
entity SHOULD use a configurable default value for both streams,
which it MUST communicate in the header for the response stream.
For all stanzas sent over the initial stream, if the initiating
entity does not include an 'xml:lang' attribute, the receiving
entity SHOULD apply the default value; if the initiating entity
does include an 'xml:lang' attribute, the receiving entity MUST
NOT modify or delete it (see also xml:lang (Section 9.1.5)). The
value of the 'xml:lang' attribute MUST be an NMTOKEN (as defined
in Section 2.3 of [XML]) and MUST conform to the format defined in
RFC 3066 [LANGTAGS].
o version -- The presence of the version attribute set to a value of
"1.0" signals support for the stream-related protocols (including
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
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---------+---------------------------+-----------------------
to | hostname of receiver | silently ignored
from | silently ignored | hostname of receiver
id | silently ignored | session key
xml:lang | default language | default language
version | signals XMPP 1.0 support | signals XMPP 1.0 support
4.2.1 Version Support
The following rules apply to the generation and handling of the
'version' attribute:
1. If the initiating entity complies with the stream-related
protocols defined herein (including Use of TLS (Section 5), Use
of SASL (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".
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 streams namespace declaration
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and a default namespace declaration (as "namespace declaration" is
defined in the XML namespaces specification [XML-NAMES]). For
detailed information regarding the streams namespace and default
namespace, see Namespace Names and Prefixes (Section 11.2).
4.4 Stream Features
If the initiating entity includes the 'version' attribute set to a
value of "1.0" in the initial stream header, the receiving entity
MUST send a <features/> child element (prefixed by the streams
namespace prefix) to the initiating entity in order to announce any
stream-level features that can be negotiated (or capabilities that
otherwise need to be advertised). Currently this is used only to
advertise Use of TLS (Section 5), Use of SASL (Section 6), and
Resource Binding (Section 7) as defined herein, and for Session
Establishment as defined in [XMPP-IM]; however, the stream features
functionality could be used to advertise other negotiable features in
the future. If an entity does not understand or support some
features, it SHOULD silently ignore them.
4.5 Stream Security
When negotiating XML streams in XMPP 1.0, TLS SHOULD be used as
defined under Use of TLS (Section 5) and SASL MUST be used as defined
under Use of SASL (Section 6). If the initiating entity attempts to
send an XML Stanza (Section 9) before the stream has been
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 streams namespace prefix. The error child MUST be
sent by a compliant entity (usually a server rather than a client) if
it perceives that a stream-level error has occurred.
4.6.1 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
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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]
</stream:error>
The <error/> element:
o MUST contain a child element corresponding to one of the defined
stanza error conditions defined below; this element MUST be
qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace
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 a user, but MAY
be shown in addition to the error message associated with the
included condition element (or elements).
4.6.3 Defined Conditions
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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 XML Namespace Names and Prefixes
(Section 11.2)).
o <conflict/> -- the server is closing the active stream for this
entity because a new stream has been initiated that conflicts with
the existing stream.
o <connection-timeout/> -- the entity has not generated any traffic
over the stream for some period of time (configurable according to
a local service policy).
o <host-gone/> -- the value of the 'to' attribute provided by the
initiating entity in the stream header corresponds to a hostname
that is no longer hosted by the server.
o <host-unknown/> -- the value of the 'to' attribute provided by the
initiating entity in the stream header does not correspond to a
hostname that is hosted by the server.
o <improper-addressing/> -- a stanza sent between two servers lacks
a 'to' or 'from' attribute (or the attribute has no value).
o <internal-server-error/> -- the server has experienced a
misconfiguration or an otherwise-undefined internal error that
prevents it from servicing the stream.
o <invalid-from/> -- the JID or hostname provided in a 'from'
address does not match an authorized JID or validated domain
negotiated between servers via SASL or dialback, or between a
client and a server via authentication and resource authorization.
o <invalid-id/> -- the stream ID or dialback ID is invalid or does
not match an ID previously provided.
o <invalid-namespace/> -- the streams namespace name is something
other than "http://etherx.jabber.org/streams" or the dialback
namespace name is something other than "jabber:server:dialback"
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(see XML Namespace Names and Prefixes (Section 11.2)).
o <invalid-xml/> -- the entity has sent invalid XML over the stream
to a server that performs validation (see Validation (Section
11.3)).
o <not-authorized/> -- the entity has attempted to send data before
the stream has been authenticated, or otherwise is not authorized
to perform an action related to stream negotiation; the receiving
entity MUST NOT process the offending stanza before sending the
stream error.
o <policy-violation/> -- the entity has violated some local service
policy; the server MAY choose to specify the policy in the <text/>
element.
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 lacks the system resources
necessary to service the stream.
o <restricted-xml/> -- the entity has attempted to send restricted
XML features such as a comment, processing instruction, DTD,
entity reference, or unescaped character (see Restrictions
(Section 11.1)).
o <see-other-host/> -- the server will not provide service to the
initiating entity but is redirecting traffic to another host; the
server SHOULD specify the alternate hostname or IP address in the
CDATA of the <see-other-host/> element.
o <system-shutdown/> -- the server is being shut down and all active
streams are being closed.
o <undefined-condition/> -- the error condition is not one of those
defined by the other conditions in this list; this error condition
SHOULD be used only in conjunction with an application-specific
condition.
o <unsupported-encoding/> -- the initiating entity has encoded the
stream in an encoding that is not supported by the server (see
Character Encoding (Section 11.5)).
o <unsupported-stanza-type/> -- the initiating entity has sent a
first-level child of the stream that is not supported by the
server.
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o <unsupported-version/> -- the value of the 'version' attribute
provided by the initiating entity in the stream header specifies a
version of XMPP that is not supported by the server; the server
MAY specify the version(s) it supports in the <text/> element.
o <xml-not-well-formed/> -- the initiating entity has sent XML that
is not well-formed as defined by [XML].
4.6.4 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
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 Simplified Stream Examples
This section contains two simplified examples of a stream-based
"session" of a client on a server (where the "C" lines are sent from
the client to the server, and the "S" lines are sent from the server
to the client); these examples are included for the purpose of
illustrating the concepts introduced thus far.
A basic "session":
C: <?xml version='1.0'?>
<stream:stream
to='example.com'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='example.com'
id='someid'
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xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
... encryption, authentication, and resource binding ...
C: <message from='juliet@example.com'
to='romeo@example.net'
xml:lang='en'>
C: <body>Art thou not Romeo, and a Montague?</body>
C: </message>
S: <message from='romeo@example.net'
to='juliet@example.com'
xml:lang='en'>
S: <body>Neither, fair saint, if either thee dislike.</body>
S: </message>
C: </stream:stream>
S: </stream:stream>
A "session" gone bad:
C: <?xml version='1.0'?>
<stream:stream
to='example.com'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='example.com'
id='someid'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
... encryption, authentication, and resource binding ...
C: <message xml:lang='en'>
<body>Bad XML, no closing body tag!
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
S: </stream:stream>
5. Use of TLS
5.1 Overview
XMPP includes a method for securing the stream from tampering and
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eavesdropping. This channel encryption method makes use of the
Transport Layer Security (TLS) protocol [TLS], along with a
"STARTTLS" extension that is modelled after similar extensions for
the IMAP [IMAP], POP3 [POP3], and ACAP [ACAP] protocols as described
in RFC 2595 [USINGTLS]. The namespace name for the STARTTLS
extension is 'urn:ietf:params:xml:ns:xmpp-tls'.
An administrator of a given domain MAY require the use of TLS for
client-to-server communications, server-to-server communications, or
both. Clients SHOULD use TLS to secure the streams prior to
attempting to complete SASL negotiation (Section 6), and servers
SHOULD use TLS between two domains for the purpose of securing
server-to-server communications.
The following rules apply:
1. An initiating entity that complies with this specification MUST
include the 'version' attribute set to a value of "1.0" in the
initial stream header.
2. If the TLS negotiation occurs between two servers,
communications MUST NOT proceed until the Domain Name System
(DNS) hostnames asserted by the servers have been resolved (see
Server-to-Server Communications (Section 14.3)).
3. When a receiving entity that complies with this specification
receives an initial stream header that includes the 'version'
attribute set to a value of "1.0", after sending a stream header
in reply (including the version flag) it MUST include a
<starttls/> element (qualified by the
'urn:ietf:params:xml:ns:xmpp-tls' namespace) along with the list
of other stream features it supports.
4. If the initiating entity chooses to use TLS, TLS negotiation
MUST be completed before proceeding to SASL negotiation; this
order of negotiation is required in order to help safeguard
authentication information sent during SASL negotiation, as well
as to make it possible to base the use of the SASL EXTERNAL
mechanism on a certificate provided during prior TLS
negotiation.
5. During TLS negotiation, an entity MUST NOT send any white space
characters (matching production [3] content of [XML]) within the
root stream element as separators between elements (any white
space characters shown in the TLS examples below are included
for the sake of readability only); this prohibition helps to
ensure proper security layer byte precision.
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6. The receiving entity MUST consider the TLS negotiation to have
begun immediately after sending the closing ">" character of the
<proceed/> element. The initiating entity MUST consider the TLS
negotiation to have begun immediately after receiving the
closing ">" character of the <proceed/> element from the
receiving entity.
7. The initiating entity MUST validate the certificate presented by
the receiving entity; there are two cases:
8.
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 [HTTP-TLS]. 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 defined under High
Security (Section 14.1)). 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 14.1) for details).
9. 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. Note well
that certificates MUST be checked against the hostname as
provided by the initiating entity (e.g., a user), not the
hostname as resolved via the Domain Name System; e.g., if a user
typed "example.com" but a DNS SRV [SRV] lookup returned
"im.example.com", the certificate MUST be checked as
"example.com".
10. If the TLS negotiation is successful, the receiving entity MUST
discard any knowledge obtained in an insecure manner from the
initiating entity before TLS takes effect.
11. If the TLS negotiation is successful, the initiating entity MUST
discard any knowledge obtained in an insecure manner from the
receiving entity before TLS takes effect.
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12. 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.
13. If the TLS negotiation is successful, the initiating entity MUST
continue with SASL negotiation.
14. If the TLS negotiation results in failure, the receiving entity
MUST terminate both the XML stream and the underlying TCP
connection.
15. See Mandatory-to-Implement Technologies (Section 14.6) regarding
mechanisms that MUST be supported.
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,
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the entities MUST attempt to complete the TLS negotiation over
the TCP connection and MUST NOT send any further XML data until
the TLS negotiation is complete.
6. The initiating entity and receiving entity attempt to complete a
TLS negotiation in accordance with [TLS].
7. If the TLS negotiation is unsuccessful, the receiving entity MUST
terminate the TCP connection (it is not necessary to send a
closing </stream> tag first, since the receiving entity and
initiating entity MUST consider the original stream to be closed
upon sending or receiving the <success/> element). 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 available features
(but NOT including the STARTTLS feature).
5.3 Client-to-Server Example
The following example shows the data flow for a client securing a
stream using STARTTLS (note: the alternate steps shown below are
provided to illustrate the protocol for failure cases; they are not
exhaustive and would not necessarily be triggered by the data sent in
the example).
Step 1: Client initiates stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 2: Server responds by sending a stream tag to client:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_123'
from='example.com'
version='1.0'>
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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'/>
</stream:stream>
Step 6: Client and server attempt to complete TLS negotiation over
the existing TCP connection.
Step 7: If TLS negotiation is successful, client initiates a new
stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP
connection.
Step 8: Server responds by sending a stream header to client along
with any available stream features:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
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from='example.com'
id='c2s_234'
version='1.0'>
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>
Step 9: Client continues with SASL negotiation (Section 6).
5.4 Server-to-Server Example
The following example shows the data flow for two servers securing a
stream using STARTTLS (note: the alternate steps shown below are
provided to illustrate the protocol for failure cases; they are not
exhaustive and would not necessarily be triggered by the data sent in
the example).
Step 1: Server1 initiates stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 2: Server2 responds by sending a stream tag to Server1:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='example.com'
id='s2s_123'
version='1.0'>
Step 3: Server2 sends the STARTTLS extension to Server1 along with
authentication mechanisms and any other stream features:
<stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
<required/>
</starttls>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
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</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'
to='example.com'
version='1.0'>
Step 7 (alt): If TLS negotiation is unsuccessful, server closes TCP
connection.
Step 8: Server2 responds by sending a stream header to Server1 along
with any available stream features:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='example.com'
id='s2s_234'
version='1.0'>
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>
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Step 9: Server1 continues with SASL negotiation (Section 6).
6. Use of SASL
6.1 Overview
XMPP includes a method for authenticating a stream by means of an
XMPP-specific profile of the Simple Authentication and Security Layer
(SASL) protocol [SASL]. SASL provides a generalized method for
adding authentication support to connection-based protocols, and XMPP
uses a generic XML namespace profile for SASL that conforms to the
profiling requirements of [SASL].
The following rules apply:
1. If the SASL negotiation occurs between two servers,
communications MUST NOT proceed until the Domain Name System
(DNS) hostnames asserted by the servers have been resolved (see
Server-to-Server Communications (Section 14.3)).
2. If the initiating entity is capable of SASL negotiation, it MUST
include the 'version' attribute set to a value of "1.0" in the
initial stream header.
3. If the receiving entity is capable of SASL negotiation, it MUST
send one or more authentication mechanisms within a <mechanisms/
> element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace in reply to the opening stream tag received from the
initiating entity (if the opening stream tag included the
'version' attribute set to a value of "1.0").
4. During SASL negotiation, an entity MUST NOT send any white space
characters (matching production [3] content of [XML]) within the
root stream element as separators between elements (any white
space characters shown in the SASL examples below are included
for the sake of readability only); this prohibition helps to
ensure proper security layer byte precision.
5. Any character data contained within the XML elements used during
SASL negotiation MUST be encoded using base64, where the
encoding adheres to the definition in Section 3 of RFC 3548
[BASE64].
6. If provision of a "simple username" is supported by the selected
SASL mechanism (e.g., this is supported by the DIGEST-MD5 and
CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI
mechanisms), during authentication the initiating entity SHOULD
provide its sending domain (in the case of server-to-server
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communications) or registered account name (in the case of
client-to-server communications) as the simple username.
7. If the initiating entity wishes to act on behalf of another
entity and the selected SASL mechanism supports transmission of
an authorization identity, the initiating entity MUST provide an
authorization identity during SASL negotiation. If the
initiating entity does not wish to act on behalf of another
entity, it MUST NOT provide an authorization identity. As
specified in [SASL], the initiating entity MUST NOT provide an
authorization identity unless the authorization identity is
different from the default authorization identity derived from
the authentication identity as described in [SASL]. If
provided, the value of the authorization identity MUST be of the
form <domain> (i.e., a domain identifier only) for servers and
of the form <node@domain> (i.e., node identifier and domain
identifier) for clients.
8. Upon successful SASL negotiation that involves negotiation of a
security layer, the receiving entity MUST discard any knowledge
obtained from the initiating entity which was not obtained from
the SASL negotiation itself.
9. Upon successful SASL negotiation that involves negotiation of a
security layer, the initiating entity MUST discard any knowledge
obtained from the receiving entity which was not obtained from
the SASL negotiation itself.
10. See Mandatory-to-Implement Technologies (Section 14.6) regarding
mechanisms that MUST be supported.
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 reply, 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 Use
of TLS (Section 5) needs to be established before a particular
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authentication mechanism may be used, the receiving entity MUST
NOT provide that mechanism in the list of available SASL
authentication mechanisms prior to TLS negotiation. If the
initiating entity presents a valid certificate during prior TLS
negotiation, the receiving entity SHOULD offer the SASL EXTERNAL
mechanism to the initiating entity during SASL negotiation (refer
to [SASL]), although the EXTERNAL mechanism MAY be offered under
other circumstances as well.
3. The initiating entity selects a mechanism by sending an <auth/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace to the receiving entity and including an appropriate
value for the 'mechanism' attribute; this element MAY contain
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 and presented a
certificate during prior TLS negotiation, the authentication
credentials SHOULD be taken from that certificate.
4. If necessary, the receiving entity challenges the initiating
entity by sending a <challenge/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
entity; this element MAY contain character data (which MUST be
computed in accordance with the definition of the SASL mechanism
chosen by the initiating entity).
5. The initiating entity responds to the challenge by sending a
<response/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the receiving
entity; this element MAY contain character data (which MUST be
computed in accordance with the definition of the SASL mechanism
chosen by the initiating entity).
6. If necessary, the receiving entity sends more challenges and the
initiating entity sends more responses.
This series of challenge/response pairs continues until one of three
things happens:
1. The initiating entity aborts the handshake by sending an <abort/>
element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
namespace to the receiving entity. Upon receiving an <abort/>
element, the receiving entity SHOULD allow a configurable but
reasonable number of retries (at least 2), after which it MUST
terminate the TCP connection; this allows the initiating entity
(e.g., an end-user client) to tolerate incorrectly-provided
credentials (e.g., a mistyped password) without being forced to
reconnect.
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2. The receiving entity reports failure of the handshake by sending
a <failure/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
entity (the particular cause of failure SHOULD be communicated in
an appropriate child element of the <failure/> element as defined
under SASL Errors (Section 6.4)). If the failure case occurs,
the receiving entity SHOULD allow a configurable but reasonable
number of retries (at least 2), after which it MUST terminate the
TCP connection; this allows the initiating entity (e.g., an
end-user client) to tolerate incorrectly-provided credentials
(e.g., a mistyped password) without being forced to reconnect.
3. The receiving entity reports success of the handshake by sending
a <success/> element qualified by the
'urn:ietf:params:xml:ns:xmpp-sasl' namespace to the initiating
entity; this element MAY contain character data (in SASL
terminology, "additional data with success") if required by the
chosen SASL mechanism. Upon receiving the <success/> element,
the initiating entity MUST initiate a new stream by sending an
opening XML stream header to the receiving entity (it is not
necessary to send a closing </stream> tag first, since the
receiving entity and initiating entity MUST consider the original
stream to be closed upon sending or receiving the <success/>
element). Upon receiving the new stream header from the
initiating entity, the receiving entity MUST respond by sending a
new XML stream header to the initiating entity, along with any
available features (but NOT including the STARTTLS feature) or an
empty <features/> element (to signify that no additional features
are available); any such additional features not defined herein
MUST be defined by the relevant extension to XMPP.
6.3 SASL Definition
The profiling requirements of [SASL] require that the following
information be supplied by a protocol definition:
service name: "xmpp"
initiation sequence: After the initiating entity provides an opening
XML stream header and the receiving entity replies in kind, the
receiving entity provides a list of acceptable authentication
methods. The initiating entity chooses one method from the list
and sends it to the receiving entity as the value of the
'mechanism' attribute possessed by an <auth/> element, optionally
including an initial response to avoid a round trip.
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exchange sequence: Challenges and responses are carried through the
exchange of <challenge/> elements from receiving entity to
initiating entity and <response/> elements from initiating entity
to receiving entity. The receiving entity reports failure by
sending a <failure/> element and success by sending a <success/>
element; the initiating entity aborts the exchange by sending an
<abort/> element. Upon successful negotiation, both sides
consider the original XML stream to be closed and new stream
headers are sent by both entities.
security layer negotiation: The security layer takes effect
immediately after sending the closing ">" character of the
<success/> element for the receiving entity, and immediately after
receiving the closing ">" character of the <success/> element for
the initiating entity. The order of layers is first [TCP], then
[TLS], then [SASL], then XMPP.
use of the authorization identity: The authorization identity may be
used by xmpp to denote the <node@domain> of a client or the
sending <domain> of a server.
6.4 SASL Errors
The following SASL-related error conditions are defined:
o <aborted/> -- The receiving entity acknowledges an <abort/>
element sent by the initiating entity; sent in reply to the
<abort/> element.
o <incorrect-encoding/> -- The data provided by the initiating
entity could not be processed because the [BASE64] encoding is
incorrect (e.g., because the encoding does not adhere to the the
definition in Section 3 of [BASE64]); sent in reply to a
<response/> element or an <auth/> element with initial challenge
data.
o <invalid-authzid/> -- The authzid provided by the initiating
entity is invalid, either because it is incorrectly formatted or
because the initiating entity does not have permissions to
authorize that ID; sent in reply to a <response/> element or an
<auth/> element with initial challenge data.
o <invalid-mechanism/> -- The initiating entity did not provide a
mechanism or requested a mechanism that is not supported by the
receiving entity; sent in reply to an <auth/> element.
o <mechanism-too-weak/> -- The mechanism requested by the initiating
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entity is weaker than server policy permits for that initiating
entity; sent in reply to a <response/> element or an <auth/>
element with initial challenge data.
o <not-authorized/> -- The authentication failed because the
initiating entity did not provide valid credentials (this includes
but is not limited to the case of an unknown username); sent in
reply to a <response/> element or an <auth/> element with initial
challenge data.
o <temporary-auth-failure/> -- The authentication failed because of
a temporary error condition within the receiving entity; sent in
reply to an <auth/> element or <response/> element.
6.5 Client-to-Server Example
The following example shows the data flow for a client authenticating
with a server using SASL, normally after successful TLS negotiation
(note: the alternate steps shown below are provided to illustrate the
protocol for failure cases; they are not exhaustive and would not
necessarily be triggered by the data sent in the example).
Step 1: Client initiates stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 2: Server responds with a stream tag sent to client:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_234'
from='example.com'
version='1.0'>
Step 3: Server informs client of available authentication mechanisms:
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
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Step 4: Client selects an authentication mechanism:
<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
Step 5: Server sends a [BASE64] encoded challenge to client:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
</challenge>
The decoded challenge is:
realm="somerealm",nonce="OA6MG9tEQGm2hh",\
qop="auth",charset=utf-8,algorithm=md5-sess
Step 5 (alt): Server returns error to client:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>
</stream:stream>
Step 6: Client sends a [BASE64] encoded response to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK
</response>
The decoded response is:
username="somenode",realm="somerealm",\
nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
nc=00000001,qop=auth,digest-uri="xmpp/example.com",\
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
Step 7: Server sends another [BASE64] encoded challenge to client:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</challenge>
The decoded challenge is:
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rspauth=ea40f60335c427b5527b84dbabcdfffd
Step 7 (alt): Server returns error to client:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 8: Client responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9: Server informs client of successful authentication:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9 (alt): Server informs client of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 10: Client initiates a new stream to server:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 11: Server responds by sending a stream header to client along
with any additional features (or an empty features element):
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_345'
from='example.com'
version='1.0'>
<stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
</stream:features>
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6.6 Server-to-Server Example
The following example shows the data flow for a server authenticating
with another server using SASL, normally after successful TLS
negotiation (note: the alternate steps shown below are provided to
illustrate the protocol for failure cases; they are not exhaustive
and would not necessarily be triggered by the data sent in the
example).
Step 1: Server1 initiates stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 2: Server2 responds with a stream tag sent to Server1:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='example.com'
id='s2s_234'
version='1.0'>
Step 3: Server2 informs Server1 of available authentication
mechanisms:
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
</mechanisms>
</stream:features>
Step 4: Server1 selects an authentication mechanism:
<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
Step 5: Server2 sends a [BASE64] encoded challenge to Server1:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVkb21haW4iLHJlYWxtPSJzb21lcmVhbG0iLG5vbmNl
PSJPQTZNRzl0RVFHbTJoaCIscW9wPSJhdXRoIixjaGFyc2V0PXV0Zi04LGFs
Z29yaXRobT1tZDUtc2Vzcwo=
</challenge>
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The decoded challenge is:
username="somedomain",realm="somerealm",\
nonce="OA6MG9tEQGm2hh",qop="auth",\
charset=utf-8,algorithm=md5-sess
Step 5 (alt): Server2 returns error to Server1:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>
</stream:stream>
Step 6: Server1 sends a [BASE64] encoded response to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVkb21haW4iLHJlYWxtPSJzb21lcmVhbG0iLG5vbmNl
PSJPQTZNRzl0RVFHbTJoaCIsY25vbmNlPSJPQTZNSFhoNlZxVHJSayIsbmM9
MDAwMDAwMDEscW9wPWF1dGgsZGlnZXN0LXVyaT0ieG1wcC9leGFtcGxlLmNv
bSIscmVzcG9uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcs
Y2hhcnNldD11dGYtOAo=
</response>
The decoded response is:
username="somedomain",realm="somerealm",\
nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
nc=00000001,qop=auth,digest-uri="xmpp/example.com",\
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
Step 7: Server2 sends another [BASE64] encoded challenge to Server1:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</challenge>
The decoded challenge is:
rspauth=ea40f60335c427b5527b84dbabcdfffd
Step 7 (alt): Server2 returns error to Server1:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<invalid-authzid/>
</failure>
</stream:stream>
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Step 8: Server1 responds to the challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 8 (alt): Server1 aborts negotiation:
<abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9: Server2 informs Server1 of successful authentication:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9 (alt): Server2 informs Server1 of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<aborted/>
</failure>
</stream:stream>
Step 10: Server1 initiates a new stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 11: Server2 responds by sending a stream header to Server1 along
with any additional features (or an empty features element):
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
from='example.com'
id='s2s_345'
version='1.0'>
<stream:features/>
7. Resource Binding
After SASL negotiation (Section 6) with the receiving entity, the
initiating entity MAY want or need to bind a specific resource to
that stream. In general this applies only to clients: in order to
conform to the addressing format (Section 3) and stanza delivery
rules (Section 10) specified herein, there MUST be a resource
identifier associated with the <node@domain> of the client (which is
either generated by the server or provided by the client
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application); this ensures that the address for use over that stream
is a "full JID" of the form <node@domain/resource>.
Upon receiving a success indication within the SASL negotiation, the
client MUST send a new stream header to the server, to which the
server MUST respond with a stream header as well as a list of
available stream features. Specifically, if the server requires the
client to bind a resource to the stream after successful SASL
negotiation, it MUST include an empty <bind/> element qualified by
the 'urn:ietf:params:xml:ns:xmpp-bind' namespace in the stream
features list it presents to the client upon sending the header for
the response stream sent after successful SASL negotiation (but not
before):
Server advertises resource binding feature to client:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_345'
from='example.com'
version='1.0'>
<stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>
Upon being so informed that resource binding is required, the client
MUST bind a resource to the stream by sending to the server an IQ
stanza of type "set" (see IQ Semantics (Section 9.2.3)) containing
data qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace.
If the client wishes to allow the server to generate the resource
identifier on its behalf, it sends an IQ stanza of type "set" that
contains an empty <bind/> element:
Client asks server to bind a resource:
<iq type='set' id='bind_1'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</iq>
A server that supports resource binding MUST be able to generate a
resource identifier on behalf of a client. A resource identifier
generated by the server MUST be unique for that <node@domain>.
If the client wishes to specify the resource identifier, it sends an
IQ stanza of type "set" that contains the desired resource identifier
as the CDATA of a <resource/> element that is a child of the <bind/>
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element:
Client binds a resource:
<iq type='set' id='bind_2'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>someresource</resource>
</bind>
</iq>
Once the server has generated a resource identifier for the client or
accepted the resource identifier provided by the client, it MUST
return an IQ stanza of type "result" to the client, which MUST
include a <jid/> child element that specifies the full JID for the
client as determined by the server:
Server informs client of successful resource binding:
<iq type='result' id='bind_2'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>somenode@somedomain/someresource</jid>
</bind>
</iq>
A server is NOT REQUIRED to accept the resource identifier provided
by the client, and MAY override it with a resource identifier that
the server generates; in this case, the server SHOULD NOT return a
stanza error (e.g., <forbidden/>) to the client but instead SHOULD
communicate the generated resource identifier to the client in the IQ
result as shown above.
When a client supplies a resource identifier, the following stanza
error conditions are possible (see Stanza Errors (Section 9.3)):
o The provided resource identifier cannot be processed by the server
in accordance with Resourceprep (Appendix B).
o The client is not allowed to bind a resource to the stream (e.g.,
because the client has reached a limit on the number of bound
resources allowed).
o The provided resource identifier is already in use but the server
does not allow binding of multiple resources with the same
identifier.
The protocol for these error conditions is shown below.
Resource identifier cannot be processed:
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<iq type='error' id='bind_2'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>someresource</resource>
</bind>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
Client is not allowed to bind a resource:
<iq type='error' id='bind_2'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>someresource</resource>
</bind>
<error type='cancel'>
<not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
Resource identifier is in use:
<iq type='error' id='bind_2'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>someresource</resource>
</bind>
<error type='cancel'>
<conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
8. Server Dialback
8.1 Overview
The Jabber protocols from which XMPP was adapted include a "server
dialback" method for protecting against domain spoofing, thus making
it more difficult to spoof XML stanzas (see Server-to-Server
Communications (Section 14.3) 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 the Domain Name System
(DNS), since one server can (normally) discover the authoritative
server for a given domain.
Because dialback depends on DNS, inter-domain communications MUST NOT
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proceed until the Domain Name System (DNS) hostnames asserted by the
servers have been resolved (see Server-to-Server Communications
(Section 14.3)).
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.
8.2 Order of Events
The following is a brief summary of the order of events in dialback:
1. Originating Server establishes a connection to Receiving Server.
2. Originating Server sends a 'key' value over the connection to
Receiving Server.
3. Receiving Server establishes a connection to Authoritative
Server.
4. Receiving Server sends the same 'key' value to Authoritative
Server.
5. Authoritative Server replies that key is valid or invalid.
6. Receiving Server informs Originating Server whether it is
authenticated or not.
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We can represent this flow of events graphically as follows:
Originating Receiving
Server Server
----------- ---------
| |
| establish connection |
| ----------------------> |
| |
| send stream header |
| ----------------------> |
| |
| send stream header |
| <---------------------- |
| | Authoritative
| send dialback key | Server
| ----------------------> | -------------
| | |
| establish connection |
| ----------------------> |
| |
| send stream header |
| ----------------------> |
| |
| send stream header |
| <---------------------- |
| |
| send verify request |
| ----------------------> |
| |
| send verify response |
| <---------------------- |
|
| report dialback result |
| <---------------------- |
| |
8.3 Protocol
The detailed protocol interaction between the servers is as follows:
1. Originating Server establishes TCP connection to Receiving
Server.
2. Originating Server sends a stream header to Receiving Server:
<stream:stream
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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; however, if Receiving Server desires to proceed, it MUST
sent a stream header back to Originating Server.
4. Originating Server sends a dialback key to Receiving Server:
<db:result
to='Receiving Server'
from='Originating Server'>
98AF014EDC0...
</db:result>
Note: This key is not examined by Receiving Server, since
Receiving Server does not keep information about Originating
Server between sessions. The key generated by Originating
Server MUST be based in part on the value of the ID provided by
Receiving Server in the previous step, and in part on a secret
shared by Originating Server and Authoritative Server. If the
value of the 'to' address does not match a hostname recognized
by Receiving Server, then Receiving Server MUST generate a
<host-unknown/> stream error condition and terminate both the
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XML stream and the underlying TCP connection. If the value of
the 'from' address matches a domain with which Receiving Server
already has an established connection, then Receiving Server
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'
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:
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<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 an <invalid-from/> stream
error condition and terminate both the XML stream and the
underlying TCP connection.
9. Authoritative Server sends a stanza back to Receiving Server
verifying whether the key was valid or invalid:
<db:verify
from='Originating Server'
to='Receiving Server'
type='valid'
id='457F9224A0...'/>
or
<db:verify
from='Originating Server'
to='Receiving Server'
type='invalid'
id='457F9224A0...'/>
Note: If the ID does not match that provided by Receiving Server
in Step 3, then Receiving Server MUST generate an <invalid-id/>
stream error condition and terminate both the XML stream and the
underlying TCP connection. If the value of the 'to' address
does not match a hostname recognized by Receiving Server, then
Receiving Server MUST generate a <host-unknown/> stream error
condition and terminate both the XML stream and the underlying
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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 an <invalid-from/> stream error
condition and terminate both the XML stream and the underlying
TCP connection. After returning the verification to Receiving
Server, Authoritative Server SHOULD terminate the stream between
them.
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 an <invalid-from/> stream error condition and terminate
both the XML stream and the underlying TCP connection. Both of these
checks help to prevent spoofing related to particular stanzas.
9. XML Stanzas
After TLS negotiation (Section 5) if desired, SASL negotiation
(Section 6), and Resource Binding (Section 7) if necessary, XML
stanzas can be sent over the streams. Three kinds of XML stanza are
defined for the 'jabber:client' and 'jabber:server' namespaces:
<message/>, <presence/>, and <iq/>. In addition, there are five
common attributes for these kinds of stanza. These common
attributes, as well as the basic semantics of the three stanza kinds,
are defined herein; more detailed information regarding the syntax of
XML stanzas in relation to instant messaging and presence
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applications is provided in [XMPP-IM].
9.1 Common Attributes
The following five attributes are common to message, presence, and IQ
stanzas:
9.1.1 to
The 'to' attribute specifies the JID of the intended recipient for
the stanza.
In the 'jabber:client' namespace, a stanza SHOULD possess a 'to'
attribute, although a stanza sent from a client to a server for
handling by that server (e.g., presence sent to the server for
broadcasting to other entities) SHOULD NOT possess a 'to' attribute.
In the 'jabber:server' namespace, a stanza MUST possess a 'to'
attribute; if a server receives a stanza that does not meet this
restriction, it MUST generate an <improper-addressing/> stream error
condition and terminate both the XML stream and the underlying TCP
connection with the offending server.
If the value of the 'to' attribute is invalid or cannot be contacted,
the entity discovering that fact (usually the sender's or recipient's
server) MUST return an appropriate error to the sender, setting the
'from' attribute of the error stanza to the value provided in the
'to' attribute of the offending stanza.
9.1.2 from
The 'from' attribute specifies the JID of the sender.
When a server receives an XML stanza within the context of an
authenticated stream qualified by the 'jabber:client' namespace, it
MUST do one of the following:
1. validate that the value of the 'from' attribute provided by the
client is that of an authorized resource for the associated
entity
2. add a 'from' address to the stanza whose value is the full JID
(<node@domain/resource>) determined by the server for the
connected resource that generated the stanza (see Determination
of Addresses (Section 3.5))
If a client attempts to send an XML stanza for which the value of the
'from' attribute does not match one of the connected resources for
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that entity, the server SHOULD return an <invalid-from/> stream error
to the client. If a client attempts to send an XML stanza over a
stream that is not yet authenticated, the server SHOULD return a
<not-authorized/> stream error to the client. If generated, both of
these conditions MUST result in closing of the stream and termination
of the underlying TCP connection; this helps to prevent a denial of
service attack launched from a rogue client.
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 (or any
validated domain) of the sending server as communicated in the SASL
negotiation or dialback negotiation; if a server receives a stanza
that does not meet this restriction, it MUST generate an
<invalid-from/> stream error condition. Both of these conditions
MUST result in closing of the stream and termination of the
underlying TCP connection; this helps to prevent a denial of service
attack launched from a rogue server.
9.1.3 id
The optional 'id' attribute MAY be used by a sending entity for
internal tracking of stanzas that it sends and receives (especially
for tracking the request-response interaction inherent in the
semantics of IQ stanzas). 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 IQ Semantics (Section 9.2.3).
9.1.4 type
The 'type' attribute specifies detailed information about the purpose
or context of the message, presence, or IQ stanza. The particular
allowable values for the 'type' attribute vary depending on whether
the stanza is a message, presence, or IQ; the values for message and
presence stanzas are specific to instant messaging and presence
applications and therefore are defined in [XMPP-IM], whereas the
values for IQ stanzas specify the role of an IQ stanza in a
structured request-response "conversation" and thus are defined under
IQ Semantics (Section 9.2.3) below. The only 'type' value common to
all three stanzas is "error", for which see Stanza Errors (Section
9.3).
9.1.5 xml:lang
A stanza SHOULD possess an 'xml:lang' attribute (as defined in
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Section 2.12 of [XML]) if the stanza contains XML character data that
is intended to be presented to a human user (as explained in RFC 2277
[CHARSET], "internationalization is for humans"). The value of the
'xml:lang' attribute specifies the default language of any such
human-readable XML character data, which MAY be overridden by the
'xml:lang' attribute of a specific child element. If a stanza does
not possess an 'xml:lang' attribute, an implementation MUST assume
that the default language is that specified for the stream as defined
under Stream Attributes (Section 4.2) above. The value of the
'xml:lang' attribute MUST be an NMTOKEN and MUST conform to the
format defined in RFC 3066 [LANGTAGS].
9.2 Basic Semantics
9.2.1 Message Semantics
The <message/> stanza kind can be seen as a "push" mechanism whereby
one entity pushes information to another entity, similar to the
communications that occur in a system such as email. All message
stanzas SHOULD possess a 'to' attribute that specifies the intended
recipient of the message; upon receiving such a stanza, a server
SHOULD route or deliver it to the intended recipient (see Server
Rules for Handling XML Stanzas (Section 10) for general routing and
delivery rules related to XML stanzas).
9.2.2 Presence Semantics
The <presence/> element can be seen as a basic broadcast or
"publish-subscribe" mechanism, whereby multiple entities receive
information (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 10) for general routing and delivery rules related to XML
stanzas, and [XMPP-IM] for presence-specific rules in the context of
an instant messaging and presence application.
9.2.3 IQ Semantics
Info/Query, or IQ, is a request-response mechanism, similar in some
ways to [HTTP]. The semantics of IQ enable an entity to make a
request of, and receive a response from, another entity. The data
content of the request and response is defined by the namespace
declaration of a direct child element of the IQ element, and the
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interaction is tracked by the requesting entity through use of the
'id' attribute. Thus IQ interactions follow a common pattern of
structured data exchange such as get/result or set/result (although
an error may be returned in reply to a request if appropriate):
Requesting Responding
Entity Entity
---------- ----------
| |
| <iq type='get' id='1'> |
| ------------------------> |
| |
| <iq type='result' id='1'> |
| <------------------------ |
| |
| <iq type='set' id='2'> |
| ------------------------> |
| |
| <iq type='error' id='2'> |
| <------------------------ |
| |
In order to enforce these semantics, the following rules apply:
1. The 'id' attribute is REQUIRED for IQ stanzas.
2. The 'type' attribute is REQUIRED for IQ stanzas. The value MUST
be one of the following:
3.
* get -- The stanza is a request for information or
requirements.
* set -- The stanza provides required data, sets new values, or
replaces existing values.
* result -- The stanza is a response to a successful get or set
request.
* error -- An error has occurred regarding processing or
delivery of a previously-sent get or set (see Stanza Errors
(Section 9.3)).
4. 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).
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5. 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).
6. An IQ stanza of type "get" or "set" MUST contain one and only one
child element (properly-namespaced as defined in [XMPP-IM]) that
specifies the semantics of the particular request or response.
7. An IQ stanza of type "result" MUST include zero or one child
elements.
8. An IQ stanza of type "error" SHOULD include the child element
contained in the associated "get" or "set" and MUST include an
<error/> child; for details, see Stanza Errors (Section 9.3).
9.3 Stanza Errors
Stanza-related errors are handled in a manner similar to stream
errors (Section 4.6). However, stanza errors are not unrecoverable,
as stream errors are; therefore error stanzas include hints regarding
actions that the original sender can take in order to remedy the
error.
9.3.1 Rules
The following rules apply to stanza-related errors:
o The receiving or processing entity that detects an error condition
in relation to a stanza MUST return to the sending entity a stanza
of the same kind (message, presence, or IQ) whose 'type' attribute
is set to a value of "error" (such a stanza is called an "error
stanza" herein).
o The entity that generates an error stanza SHOULD (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 An error stanza MUST contain an <error/> child element.
o An <error/> child MUST NOT be included if the 'type' attribute has
a value other than "error" (or if there is no 'type' attribute).
o An entity that receives an error stanza MUST NOT respond to the
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stanza with a further error stanza; this helps to prevent looping.
9.3.2 Syntax
The syntax for stanza-related errors is as follows:
<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 specified 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.
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The <text/> element is OPTIONAL. If included, it SHOULD be used only
to provide descriptive or diagnostic information that supplements the
meaning of a defined condition or application-specific condition. It
SHOULD NOT be interpreted programmatically by an application. It
SHOULD NOT be used as the error message presented to a user, but MAY
be shown in addition to the error message associated with the
included condition element (or elements).
Finally, to maintain backward compatibility, the schema (specified in
[XMPP-IM]) allows the optional inclusion of a 'code' attribute on the
<error/&gr; element.
9.3.3 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., an IQ stanza that includes an
unrecognized value of the 'type' attribute); the associated error
type SHOULD be "modify".
o <conflict/> -- access cannot be granted because an existing
resource or session exists with the same name or address; the
associated error type SHOULD be "cancel".
o <feature-not-implemented/> -- the feature requested is not
implemented by the recipient or server and therefore cannot be
processed; the associated error type SHOULD be "cancel".
o <forbidden/> -- the requesting entity does not possess the
required permissions to perform the action; the associated error
type SHOULD be "auth".
o <gone/> -- the recipient or server can no longer be contacted at
this address (the error stanza MAY contain a new address in the
CDATA of the <gone/> element); the associated error type SHOULD be
"modify".
o <internal-server-error/> -- the server could not process the
stanza because of a misconfiguration or an otherwise-undefined
internal server error; the associated error type SHOULD be "wait".
o <item-not-found/> -- the addressed JID or item requested cannot be
found; the associated error type SHOULD be "cancel".
o <jid-malformed/> -- the value of the 'to' attribute in the
sender's stanza does not adhere to the syntax defined in
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Addressing Scheme (Section 3); the associated error type SHOULD be
"modify".
o <not-acceptable/> -- the recipient or server understands the
request but is refusing to process it because it does not meet
criteria defined by the recipient or server (e.g., a local policy
regarding acceptable words in messages); the associated error type
SHOULD be "cancel".
o <not-allowed/> -- the recipient or server does not allow any
entity to perform the action; the associated error type SHOULD be
"cancel".
o <payment-required/> -- the requesting entity is not authorized to
access the requested service because payment is required; the
associated error type SHOULD be "auth".
o <recipient-unavailable/> -- the intended recipient is temporarily
unavailable; the associated error type SHOULD be "wait" (note: an
application MUST NOT return this error if doing so would provide
information about the intended recipient's network availability to
an entity that is not authorized to know such information).
o <redirect/> -- the recipient or server is redirecting requests for
this information to another entity, usually temporarily (the error
stanza MAY contain a new address in the CDATA of the <redirect/>
element); the associated error type SHOULD be "modify".
o <registration-required/> -- the requesting entity is not
authorized to access the requested service because registration is
required; the associated error type SHOULD be "auth".
o <remote-server-not-found/> -- a remote server or service specified
as part or all of the JID of the intended recipient does not
exist; the associated error type SHOULD be "cancel".
o <remote-server-timeout/> -- a remote server or service specified
as part or all of the JID of the intended recipient could not be
contacted within a reasonable amount of time; the associated error
type SHOULD be "wait".
o <resource-constraint/> -- the server or recipient lacks the system
resources necessary to service the request; the associated error
type SHOULD be "wait".
o <service-unavailable/> -- the server or recipient does not
currently provide the requested service; the associated error type
SHOULD be "cancel".
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o <subscription-required/> -- the requesting entity is not
authorized to access the requested service because a subscription
is required; the associated error type SHOULD be "auth".
o <undefined-condition/> -- the error condition is not one of those
defined by the other conditions in this list; any error type may
be associated with this condition, and it SHOULD be used only in
conjunction with an application-specific condition.
o <unexpected-request/> -- the recipient or server understood the
request but was not expecting it at this time (e.g., the request
was out of order); the associated error type SHOULD be "wait".
9.3.4 Application-Specific Conditions
As noted, an application MAY provide application-specific stanza
error information by including a properly-namespaced child in the
error element. The application-specific element SHOULD supplement or
further qualify a defined element. Thus the <error/> element will
contain two or three child elements:
<iq type='error' id='some-id'>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<too-many-parameters xmlns='application-ns'/>
</error>
</iq>
<message type='error' id='another-id'>
<error type='modify'>
<undefined-condition
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<text xml:lang='en'
xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
Some special application diagnostic information...
</text>
<special-application-condition xmlns='application-ns'/>
</error>
</message>
10. Server Rules for Handling XML Stanzas
Compliant server implementations MUST ensure in-order processing of
XML stanzas between any two entities.
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Beyond the requirement for in-order processing, each server
implementation will contain its own "delivery tree" for handling
stanzas it receives. Such a tree determines whether a stanza needs
to be routed to another domain, processed internally, or delivered to
a resource associated with a connected node. The following rules
apply:
10.1 No 'to' Address
If the stanza possesses no 'to' attribute, the server SHOULD process
it on behalf of the entity that sent it. Because all stanzas
received from other servers MUST possess a 'to' attribute, this rule
applies only to stanzas received from a registered entity (such as a
client) that is connected to the server. If the server receives a
presence stanza with no 'to' attribute, the server SHOULD broadcast
it to the entities that are subscribed to the sending entity's
presence, if applicable (the semantics of presence broadcast for
instant messaging and presence applications are defined in
[XMPP-IM]). If the server receives an IQ stanza of type "get" or
"set" with no 'to' attribute and it understands the namespace that
qualifies the content of the stanza, it MUST either process the
stanza on behalf of sending entity (where the meaning of "process" is
determined by the semantics of the qualifying namespace) or return an
error to the sending entity.
10.2 Foreign Domain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute does not match one of the configured hostnames
of the server itself or a subdomain thereof, the server SHOULD route
the stanza to the foreign domain (subject to local service
provisioning and security policies regarding inter-domain
communication). There are two possible cases:
A server-to-server stream already exists between the two domains: The
sender's server routes the stanza to the authoritative server for
the foreign domain over the existing stream
There exists no server-to-server stream between the two domains: The
sender's server (1) resolves the hostname of the foreign domain
(as defined under Server-to-Server Communications (Section 14.3)),
(2) negotiates a server-to-server stream between the two domains
(as defined under Use of TLS (Section 5) and Use of SASL (Section
6)), and (3) routes the stanza to the authoritative server for the
foreign domain over the newly-established stream
If routing to the recipient's server is unsuccessful, the sender's
server MUST return an error to the sender; if the recipient's server
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can be contacted but delivery by the recipient's server to the
recipient is unsuccessful, the recipient's server MUST return an
error to the sender by way of the sender's server.
10.3 Subdomain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute matches a subdomain of one of the configured
hostnames of the server itself, the server MUST either process the
stanza itself or route the stanza to a specialized service that is
responsible for that subdomain (if the subdomain is configured), or
return an error to the sender (if the subdomain is not configured).
10.4 Mere Domain or Specific Resource
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute matches a configured hostname of the server
itself and the JID contained in the 'to' attribute is of the form
<domain> or <domain/resource>, the server (or a defined resource
thereof) MUST either process the stanza as appropriate for the stanza
kind or return an error stanza to the sender.
10.5 Node in Same Domain
If the hostname of the domain identifier portion of the JID contained
in the 'to' attribute matches a configured hostname of the server
itself and the JID contained in the 'to' attribute is of the form
<node@domain> or <node@domain/resource>, the server SHOULD deliver
the stanza to the intended recipient of the stanza as represented by
the JID contained in the 'to' attribute. The following rules apply:
1. If the JID contains a resource identifier (i.e., is of the form
<node@domain/resource>) and there is an available resource that
matches the full JID, the recipient's server SHOULD deliver the
stanza to the stream or session that exactly matches the resource
identifier.
2. If the JID contains a resource identifier and there is no
available resource that matches the full JID, the recipient's
server SHOULD return to the sender a <service-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
defined in [XMPP-IM]).
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11. XML Usage within XMPP
11.1 Restrictions
XMPP is a simplified and specialized protocol for streaming XML
elements in order to exchange structured information in close to real
time. Because XMPP does not require the parsing of arbitrary and
complete XML documents, there is no requirement that XMPP needs to
support the full feature set of [XML]. In particular, the following
restrictions apply.
With regard to XML generation, an XMPP implementation MUST NOT inject
into an XML stream any of the following:
o comments (as defined in Section 2.5 of [XML])
o processing instructions (Section 2.6 therein)
o internal or external DTD subsets (Section 2.8 therein)
o internal or external entity references (Section 4.2 therein) with
the exception of predefined entities (Section 4.6 therein)
o character data or attribute values containing unescaped characters
that map to the predefined entities (Section 4.6 therein); such
characters MUST be escaped
With regard to XML processing, if an XMPP implementation receives
such restricted XML data, it MUST ignore the data.
11.2 XML Namespace Names and Prefixes
XML Namespaces [XML-NAMES] are used within all XMPP-compliant XML to
create strict boundaries of data ownership. The basic function of
namespaces is to separate different vocabularies of XML elements that
are structurally mixed together. Ensuring that XMPP-compliant XML is
namespace-aware enables any allowable XML to be structurally mixed
with any data element within XMPP. Rules for XML namespace names and
prefixes are defined in the following subsections.
11.2.1 Streams Namespace
A streams namespace declaration is REQUIRED in all XML stream
headers. The name of the streams namespace MUST be 'http://
etherx.jabber.org/streams'. The element names of the <stream/>
element and its <features/> and <error/> children MUST be qualified
by the streams namespace prefix in all instances. An implementation
SHOULD generate only the 'stream:' prefix for these elements, and for
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historical reasons MAY accept only the 'stream:' prefix.
11.2.2 Default Namespace
A default namespace declaration is REQUIRED and is used in all XML
streams in order to define the allowable first-level children of the
root stream element. This namespace declaration MUST be the same for
the initial stream and the response stream so that both streams are
qualified consistently. The default namespace declaration applies to
the stream and all stanzas sent within a stream (unless explicitly
qualified by another namespace, or by the prefix of the streams
namespace or the dialback namespace).
A server implementation MUST support the following two default
namespaces (for historical reasons, some implementations MAY support
only these two default namespaces):
o jabber:client -- this default namespace is declared when the
stream is used for communications between a client and a server
o jabber:server -- this default namespace is declared when the
stream is used for communications between two servers
A client implementation MUST support the 'jabber:client' default
namespace, and for historical reasons MAY support only that default
namespace.
An implementation MUST NOT generate namespace prefixes for elements
in the default namespace if the default namespace is 'jabber:client'
or 'jabber:server'. An implementation SHOULD NOT generate namespace
prefixes for elements qualified by content (as opposed to stream)
namespaces other than 'jabber:client' and 'jabber:server'.
Note: The 'jabber:client' and 'jabber:server' namespaces are nearly
identical but are used in different contexts (client-to-server
communications for 'jabber:client' and server-to-server
communications for 'jabber:server'). The only difference between the
two is that the 'to' and 'from' attributes are OPTIONAL on stanzas
sent within 'jabber:client', whereas they are REQUIRED on stanzas
sent within 'jabber:server'. If a compliant implementation accepts a
stream that is qualified by the 'jabber:client' or 'jabber:server'
namespace, it MUST support the common attributes (Section 9.1) and
basic semantics (Section 9.2) of all three core stanza kinds
(message, presence, and IQ).
11.2.3 Dialback Namespace
A dialback namespace declaration is REQUIRED for all elements used in
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server dialback (Section 8). The name of the dialback namespace MUST
be 'jabber:server:dialback'. All elements qualified by this
namespace MUST be prefixed. An implementation SHOULD generate only
the 'db:' prefix for such elements and MAY accept only the 'db:'
prefix.
11.3 Validation
Except as noted with regard to 'to' and 'from' addresses for stanzas
within the 'jabber:server' namespace, a server is not responsible for
validating the XML elements forwarded to a client or another server;
an implementation MAY choose to provide only validated data elements
but 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.
11.4 Inclusion of Text Declaration
Implementations SHOULD send a text declaration before sending a
stream header. Applications MUST follow the rules in [XML] regarding
the circumstances under which a text declaration is included.
11.5 Character Encoding
Implementations MUST support the UTF-8 (RFC 2279 [UTF-8])
transformation of Universal Character Set (ISO/IEC 10646-1 [UCS2])
characters, as required by RFC 2277 [CHARSET]. Implementations MUST
NOT attempt to use any other encoding.
12. Core Compliance Requirements
This section summarizes the specific aspects of the Extensible
Messaging and Presence Protocol that MUST be supported by servers and
clients in order to be considered compliant implementations, as well
as additional protocol aspects that SHOULD be supported. For
compliance purposes, we draw a distinction between core protocols
(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].
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12.1 Servers
In addition to all defined requirements with regard to security, XML
usage, and internationalization, a server MUST support the following
core protocols in order to be considered compliant:
o Application of the [NAMEPREP], Nodeprep (Appendix A), and
Resourceprep (Appendix B) profiles of [STRINGPREP] to addresses
(including ensuring that domain identifiers are internationalized
domain names as defined in [IDNA])
o XML streams (Section 4), including Use of TLS (Section 5), Use of
SASL (Section 6), and Resource Binding (Section 7)
o The basic semantics of the three defined stanza kinds (i.e.,
<message/>, <presence/>, and <iq/>) as specified in stanza
semantics (Section 9.2)
o Generation (and, where appropriate, handling) of error syntax and
semantics related to streams, TLS, SASL, and XML stanzas
In addition, a server SHOULD support the following core protocol:
o Server dialback (Section 8)
12.2 Clients
A client MUST support the following core protocols in order to be
considered compliant:
o XML streams (Section 4), including Use of TLS (Section 5), Use of
SASL (Section 6), and Resource Binding (Section 7)
o The basic semantics of the three defined stanza kinds (i.e.,
<message/>, <presence/>, and <iq/>) as specified in stanza
semantics (Section 9.2)
o Handling (and, where appropriate, generation) of error syntax and
semantics related to streams, TLS, SASL, and XML stanzas
In addition, a client SHOULD support the following core protocols:
o Generation of addresses to which the [NAMEPREP], Nodeprep
(Appendix A), and Resourceprep (Appendix B) profiles of
[STRINGPREP] can be applied without failing
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13. Internationalization Considerations
XML streams MUST be encoded in UTF-8 as specified under Character
Encoding (Section 11.5). As specified under Stream Attributes
(Section 4.2), an XML stream SHOULD include an 'xml:lang' attribute
that is treated as the default language for any XML character data
sent over the stream that is intended to be presented to a human
user. As specified under xml:lang (Section 9.1.5), an XML stanza
SHOULD include an 'xml:lang' attribute if the stanza contains XML
character data that is intended to be presented to a human user. A
server SHOULD apply the default 'xml:lang' attribute to stanzas it
routes or delivers on behalf of connected entities, and MUST NOT
modify or delete 'xml:lang' attributes from stanzas it receives from
other entities.
14. Security Considerations
14.1 High Security
For the purposes of XMPP communications (client-to-server and
server-to-server), the term "high security" refers to the use of
security technologies that provide both mutual authentication and
integrity-checking; in particular, when using certificate-based
authentication to provide high security, a chain-of-trust SHOULD be
established out-of-band, although a shared certificate authority
signing certificates could allow a previously unknown certificate to
establish trust in-band.
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.
14.2 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 Use of TLS
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(Section 5)) provides a reliable mechanism for helping to ensure the
confidentiality and data integrity of data exchanged between two
entities.
The SASL protocol for authenticating XML streams (defined under Use
of SASL (Section 6)) provides a reliable mechanism for validating
that a client connecting to a server is who it claims to be.
Client-to-server communications MUST NOT proceed until the DNS
hostname asserted by the server has been resolved. Such resolutions
SHOULD first attempt to resolve the hostname using an [SRV] Service
of "xmpp-client" and Proto of "tcp", resulting in resource records
such as "_xmpp-client._tcp.example.com." (the use of the string
"xmpp-client" for the service identifier is consistent with the IANA
registration). If the SRV lookup fails, the fallback is a normal
IPv4/IPv6 address record resolution to determine the IP address,
using the "xmpp-client" port of 5222 registered with the IANA.
The IP address and method of access of clients MUST NOT be made
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.
14.3 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 8).
Because service provisioning is a matter of policy, it is OPTIONAL
for any given domain to communicate with other domains, and
server-to-server communications MAY be disabled by the administrator
of any given deployment. If a particular domain enables inter-domain
communications, it SHOULD enable high security.
Administrators may want to require use of SASL for server-to-server
communications in order to ensure both authentication and
confidentiality (e.g., on an organization's private network).
Compliant implementations SHOULD support SASL for this purpose.
Inter-domain connections MUST NOT proceed until the DNS hostnames
asserted by the servers have been resolved. Such resolutions MUST
first attempt to resolve the hostname using an [SRV] Service of
"xmpp-server" and Proto of "tcp", resulting in resource records such
as "_xmpp-server._tcp.example.com." (the use of the string
"xmpp-server" for the service identifier is consistent with the IANA
registration; note well that the "xmpp-server" service identifier
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supersedes the earlier use of a "jabber" service identifier, since
the earlier usage did not conform to [SRV]; implementations desiring
to be backward compatible should continue to look for or answer to
the "jabber" service identifier as well). If the SRV lookup fails,
the fallback is a normal IPv4/IPv6 address record resolution to
determine the IP address, using the "xmpp-server" port of 5269
registered with the IANA.
Server dialback helps protect against domain spoofing, thus making it
more difficult to spoof XML stanzas. It is not a mechanism for
authenticating, securing, or encrypting streams between servers as is
done via SASL and TLS. Furthermore, it is susceptible to DNS
poisoning attacks unless DNSSec [DNSSEC] is used, and even if the DNS
information is accurate, dialback cannot protect from attacks where
the attacker is capable of hijacking the IP address of the remote
domain. Domains requiring robust security SHOULD use TLS and SASL.
If SASL is used for server-to-server authentication, dialback SHOULD
NOT be used since it is unnecessary.
14.4 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.
14.5 Lack of SASL Channel Binding to TLS
The SASL framework does not provide a mechanism to bind SASL
authentication to a security layer providing confidentiality and
integrity protection that was negotiated at a lower layer. This lack
of a "channel binding" prevents SASL from being able to verify that
the source and destination end points to which the lower layer's
security is bound are equivalent to the end points that SASL is
authenticating. If the end points are not identical, the lower
layer's security cannot be trusted to protect data transmitted
between the SASL authenticated entities. In such a situation, a SASL
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security layer should be negotiated which effectively ignores the
presence of the lower layer security.
14.6 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)
14.7 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 (see IANA Considerations (Section 15)). Use
of these well-known ports allows administrators to easily enable or
disable XMPP activity through existing and commonly-deployed
firewalls.
14.8 Use of base64 in SASL
Both the client and the server MUST verify any [BASE64] data received
during SASL negotiation. An implementation MUST reject (not ignore)
any characters that are not explicitly allowed by the base64
alphabet; this helps to guard against creation of a covert channel
that could be used to "leak" information. An implementation MUST NOT
break on invalid input and MUST reject any sequence of base64
characters containing the pad ('=') character if that character is
included as something other than the last character of the data (e.g.
"=AAA" or "BBBB=CCC"); this helps to guard against buffer overflow
attacks and other attacks on the implementation. Base encoding
visually hides otherwise easily recognized information, such as
passwords, but does not provide any computational confidentiality.
Base 64 encoding MUST follow the definition in Section 3 of RFC 3548
[BASE64].
14.9 Stringprep Profiles
XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for
processing of domain identifiers; for security considerations related
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to Nameprep, refer to the appropriate section of [NAMEPREP].
In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
(Appendix A) for node identifiers and Resourceprep (Appendix B) for
resource identifiers.
The Unicode and ISO/IEC 10646 repertoires have many characters that
look similar. In many cases, users of security protocols might do
visual matching, such as when comparing the names of trusted third
parties. Because it is impossible to map similar-looking characters
without a great deal of context such as knowing the fonts used,
stringprep does nothing to map similar-looking characters together
nor to prohibit some characters because they look like others.
A node identifier can be employed as one part of an entity's address
in XMPP. One common usage is as the username of an instant messaging
user; another is as the name of a multi-user chat room; 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.
15. IANA Considerations
15.1 XML Namespace Name for TLS Data
A URN sub-namespace for TLS-related data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in The IETF XML Registry
[XML-REG].)
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URI: urn:ietf:params:xml:ns:xmpp-tls
Specification: XXXX
Description: This is the XML namespace name for TLS-related data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
15.2 XML Namespace Name for SASL Data
A URN sub-namespace for SASL-related data in the Extensible Messaging
and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-sasl
Specification: XXXX
Description: This is the XML namespace name for SASL-related data in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
15.3 XML Namespace Name for Stream Errors
A URN sub-namespace for stream-related error data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-streams
Specification: XXXX
Description: This is the XML namespace name for stream-related error
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
15.4 XML Namespace Name for Resource Binding
A URN sub-namespace for resource binding in the Extensible Messaging
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and Presence Protocol (XMPP) is defined as follows. (This namespace
name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-bind
Specification: XXXX
Description: This is the XML namespace name for resource binding in
the Extensible Messaging and Presence Protocol (XMPP) as defined
by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
15.5 XML Namespace Name for Stanza Errors
A URN sub-namespace for stanza-related error data in the Extensible
Messaging and Presence Protocol (XMPP) is defined as follows. (This
namespace name adheres to the format defined in [XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-stanzas
Specification: XXXX
Description: This is the XML namespace name for stanza-related error
data in the Extensible Messaging and Presence Protocol (XMPP) as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
15.6 Nodeprep Profile of Stringprep
The Nodeprep profile of stringprep is defined under Nodeprep
(Appendix A). The IANA registers Nodeprep in the stringprep profile
registry.
Name of this profile:
Nodeprep
RFC in which the profile is defined:
XXXX
Indicator whether or not this is the newest version of the profile:
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This is the first version of Nodeprep
15.7 Resourceprep Profile of Stringprep
The Resourceprep profile of stringprep is defined under Resourceprep
(Appendix B). The IANA registers Resourceprep in the stringprep
profile registry.
Name of this profile:
Resourceprep
RFC in which the profile is defined:
XXXX
Indicator whether or not this is the newest version of the profile:
This is the first version of Resourceprep
15.8 GSSAPI Service Name
The IANA registers "xmpp" as a GSSAPI [GSS-API] service name, as
defined under SASL Definition (Section 6.3).
15.9 Port Numbers
The IANA currently registers "jabber-client" and "jabber-server" as
keywords for [TCP] ports 5222 and 5269 respectively. The IANA shall
change these registrations to "xmpp-client" and "xmpp-server"
respectively.
These ports SHOULD be used for client-to-server and server-to-server
communications respectively, but their use is NOT REQUIRED.
Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[CHARSET] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
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[GSS-API] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, January 1997.
[HTTP-TLS]
Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[IMP-REQS]
Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[LANGTAGS]
Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[IDNA] Faltstrom, P., Hoffman, P. and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[NAMEPREP]
Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003.
[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
[SRV] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[STRINGPREP]
Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("STRINGPREP")", RFC 3454,
December 2002.
[TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[TLS] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A.
and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,
January 1999.
[UCS2] International Organization for Standardization,
"Information Technology - Universal Multiple-octet coded
Character Set (UCS) - Amendment 2: UCS Transformation
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Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2,
October 1996.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
"Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.
[XML-NAMES]
Bray, T., Hollander, D. and A. Layman, "Namespaces in
XML", W3C REC-xml-names, January 1999, <http://www.w3.org/
TR/REC-xml-names>.
Informative References
[ACAP] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.
[DNSSEC] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[IMAP] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, December 1996.
[JSF] Jabber Software Foundation, "Jabber Software Foundation",
<http://www.jabber.org/>.
[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[URI] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[USINGTLS]
Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
[XML-REG] Mealling, M., "The IETF XML Registry",
draft-mealling-iana-xmlns-registry-05 (work in progress),
June 2003.
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[XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
draft-ietf-xmpp-im-19 (work in progress), November 2003.
Author's Address
Peter Saint-Andre
Jabber Software Foundation
EMail: stpeter@jabber.org
Appendix A. Nodeprep
A.1 Introduction
This appendix defines the "Nodeprep" profile of [STRINGPREP]. As
such, it specifies processing rules that will enable users to enter
internationalized node identifiers in the Extensible Messaging and
Presence Protocol (XMPP) and have the highest chance of getting the
content of the strings correct. (An XMPP node identifier is the
optional portion of an XMPP address that precedes a domain identifier
and the '@' separator; it is often but not exclusively associated
with an instant messaging username.) These processing rules are
intended only for XMPP node identifiers and are not intended for
arbitrary text or any other aspect of an XMPP address.
This profile defines the following, as required by [STRINGPREP]:
o The intended applicability of the profile: internationalized node
identifiers within XMPP
o The character repertoire that is the input and output to
stringprep: Unicode 3.2, specified in Section 2 of this Appendix
o The mappings used: specified in Section 3
o The Unicode normalization used: specified in Section 4
o The characters that are prohibited as output: specified in Section
5
o Bidirectional character handling: specified in Section 6
A.2 Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
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being Table A.1, both defined in Appendix A of [STRINGPREP].
A.3 Mapping
This profile specifies mapping using the following tables from
[STRINGPREP]:
Table B.1
Table B.2
A.4 Normalization
This profile specifies using Unicode normalization form KC, as
described in [STRINGPREP].
A.5 Prohibited Output
This profile specifies prohibiting use of the following tables from
[STRINGPREP].
Table C.1.1
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
Table C.9
In addition, the following Unicode characters are also prohibited:
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#x22 (")
#x26 (&)
#x27 (')
#x2F (/)
#x3A (:)
#x3C (<)
#x3E (>)
#x40 (@)
A.6 Bidirectional Characters
This profile specifies checking bidirectional strings as described in
Section 6 of [STRINGPREP].
Appendix B. Resourceprep
B.1 Introduction
This appendix defines the "Resourceprep" profile of [STRINGPREP]. As
such, it specifies processing rules that will enable users to enter
internationalized resource identifiers in the Extensible Messaging
and Presence Protocol (XMPP) and have the highest chance of getting
the content of the strings correct. (An XMPP resource identifier is
the optional portion of an XMPP address that follows a domain
identifier and the '/' separator; it is often but not exclusively
associated with an instant messaging session name.) These processing
rules are intended only for XMPP resource identifiers and are not
intended for arbitrary text or any other aspect of an XMPP address.
This profile defines the following, as required by [STRINGPREP]:
o The intended applicability of the profile: internationalized
resource identifiers within XMPP
o The character repertoire that is the input and output to
stringprep: Unicode 3.2, specified in Section 2 of this Appendix
o The mappings used: specified in Section 3
o The Unicode normalization used: specified in Section 4
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o The characters that are prohibited as output: specified in Section
5
o Bidirectional character handling: specified in Section 6
B.2 Character Repertoire
This profile uses Unicode 3.2 with the list of unassigned code points
being Table A.1, both defined in Appendix A of [STRINGPREP].
B.3 Mapping
This profile specifies mapping using the following tables from
[STRINGPREP]:
Table B.1
B.4 Normalization
This profile specifies using Unicode normalization form KC, as
described in [STRINGPREP].
B.5 Prohibited Output
This profile specifies prohibiting use of the following tables from
[STRINGPREP].
Table C.1.2
Table C.2.1
Table C.2.2
Table C.3
Table C.4
Table C.5
Table C.6
Table C.7
Table C.8
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Table C.9
B.6 Bidirectional Characters
This profile specifies checking bidirectional strings as described in
Section 6 of [STRINGPREP].
Appendix C. XML Schemas
The following XML schemas are descriptive, not normative. For
schemas defining the 'jabber:client' and 'jabber:server' namespaces,
refer to [XMPP-IM].
C.1 Streams namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='http://etherx.jabber.org/streams'
xmlns='http://etherx.jabber.org/streams'
elementFormDefault='unqualified'>
<xs:import namespace='http://www.w3.org/XML/1998/namespace'
schemaLocation='http://www.w3.org/2001/xml.xsd'/>
<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 ref='xml:lang' use='optional'/>
<xs:attribute name='version' type='xs:decimal' use='optional'/>
</xs:complexType>
</xs:element>
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<xs:element name='features'>
<xs:complexType>
<xs:sequence>
<xs:any
namespace='##other'
minOccurs='0'
maxOccurs='unbounded'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='error'>
<xs:complexType>
<xs:sequence>
<xs:any namespace='urn:ietf:params:xml:ns:xmpp-streams'
maxOccurs='2'/>
<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'/>
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<xs:element name='internal-server-error' type='empty'/>
<xs:element name='invalid-from' type='empty'/>
<xs:element name='invalid-id' type='empty'/>
<xs:element name='invalid-namespace' type='empty'/>
<xs:element name='invalid-xml' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='policy-violation' type='empty'/>
<xs:element name='remote-connection-failed' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='restricted-xml' type='empty'/>
<xs:element name='see-other-host' type='xs:string'/>
<xs:element name='system-shutdown' type='empty'/>
<xs:element name='undefined-condition' type='empty'/>
<xs:element name='unsupported-encoding' type='empty'/>
<xs:element name='unsupported-stanza-type' type='empty'/>
<xs:element name='unsupported-version' type='empty'/>
<xs:element name='xml-not-well-formed' type='empty'/>
<xs: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'
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minOccurs='0'
maxOccurs='1'/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name='required' type='empty'/>
<xs:element name='proceed' type='empty'/>
<xs:element name='failure' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
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'/>
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<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='incorrect-encoding'/>
<xs:element ref='invalid-authzid'/>
<xs:element ref='invalid-mechanism'/>
<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='incorrect-encoding' type='empty'/>
<xs:element name='invalid-authzid' type='empty'/>
<xs:element name='invalid-mechanism' type='empty'/>
<xs:element name='mechanism-too-weak' type='empty'/>
<xs:element name='not-authorized' type='empty'/>
<xs:element name='temporary-auth-failure' type='empty'/>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
C.5 Resource binding namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-bind'
xmlns='urn:ietf:params:xml:ns:xmpp-bind'
elementFormDefault='qualified'>
<xs:element name='bind'>
<xs:complexType>
<xs:choice minOccurs='0' maxOccurs='1'>
<xs:element ref='resource'/>
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<xs:element ref='jid'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name='resource' type='xs:string'/>
<xs:element name='jid' type='xs:string'/>
</xs:schema>
C.6 Dialback namespace
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='jabber:server:dialback'
xmlns='jabber:server:dialback'
elementFormDefault='qualified'>
<xs:element name='result'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:NMTOKEN'>
<xs:attribute name='from' type='xs:string' use='required'/>
<xs:attribute name='to' type='xs:string' use='required'/>
<xs:attribute name='type' use='optional'>
<xs:simpleType>
<xs:restriction base='xs:NCName'>
<xs:enumeration value='invalid'/>
<xs:enumeration value='valid'/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='verify'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='xs:NMTOKEN'>
<xs:attribute name='from' type='xs:string' use='required'/>
<xs:attribute name='to' type='xs:string' use='required'/>
<xs:attribute name='id' type='xs:NMTOKEN' use='required'/>
<xs:attribute name='type' use='optional'>
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<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.7 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'/>
<xs:element name='feature-not-implemented' type='empty'/>
<xs:element name='forbidden' type='empty'/>
<xs:element name='gone' type='xs:string'/>
<xs:element name='internal-server-error' type='empty'/>
<xs:element name='item-not-found' type='empty'/>
<xs:element name='jid-malformed' type='empty'/>
<xs:element name='not-acceptable' type='empty'/>
<xs:element name='not-allowed' type='empty'/>
<xs:element name='payment-required' type='empty'/>
<xs:element name='recipient-unavailable' type='empty'/>
<xs:element name='redirect' type='xs:string'/>
<xs:element name='registration-required' type='empty'/>
<xs:element name='remote-server-not-found' type='empty'/>
<xs:element name='remote-server-timeout' type='empty'/>
<xs:element name='resource-constraint' type='empty'/>
<xs:element name='service-unavailable' type='empty'/>
<xs:element name='subscription-required' type='empty'/>
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<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>
Appendix D. Differences Between Core Jabber Protocol 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, while the corresponding section of
[XMPP-IM] summarizes the differences that relate specifically to
instant messaging and presence applications.
D.1 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 under Use of TLS (Section 5)
herein.
D.2 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 [JSF]). XMPP uses SASL for
authentication, as defined under Use of SASL (Section 6) herein.
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The Jabber community does not currently possess an authentication
protocol for server-to-server communications, only the Server
Dialback (Section 8) protocol to prevent server spoofing. XMPP
augments Server Dialback with a true server-to-server authentication
protocol, as defined under Use of SASL (Section 6) herein.
D.3 Resource Binding
Resource binding in the Jabber community is handled via the
'jabber:iq:auth' namespace that is also used for client
authentication with a server. XMPP defines a dedicated namespace for
resource binding as well as the ability for a server to generate a
resource identifier on behalf of a client, as defined under Resource
Binding (Section 7).
D.4 JID Processing
JID processing was somewhat loosely defined by the Jabber community
(documentation of forbidden characters and case handling is contained
in "JEP-0029: Definition of Jabber Identifiers", published by the
Jabber Software Foundation [JSF]). XMPP specifies the use of
[NAMEPREP] for domain identifiers and supplements Nameprep with two
additional [STRINGPREP] profiles for JID processing: Nodeprep
(Appendix A) for node identifiers and Resourceprep (Appendix B) for
resource identifiers .
D.5 Error Handling
Stream-related errors 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 under Stream
Errors (Section 4.6) herein.
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 under Stanza Errors (Section 9.3)
herein. (Documentation of a mapping between Jabber and XMPP error
handling mechanisms is contained in "JEP-0086: Legacy Errors",
published by the Jabber Software Foundation [JSF].)
D.6 Internationalization
Although use of UTF-8 has always been standard practice within the
Jabber community, the community did not define mechanisms for
specifying the language of human-readable text provided in CDATA
sections. XMPP specifies the use of the 'xml:lang' attribute in such
contexts, as defined under Stream Attributes (Section 4.2) and
xml:lang (Section 9.1.5) herein.
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D.7 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 under Version Support
(Section 4.2.1) herein.
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-19
o Fixed several typographical errors.
o Restricted values of 'type' attribute for IQ stanzas to those
defined in the schema (i.e., changed SHOULD to MUST) to ensure
consistency with text in XMPP IM.
o Added reference to RFC 3548.
o Replaced RFC 2222 reference with reference to
draft-ietf-sasl-rfc2222bis.
o Further clarified role and usage of user names in SASL mechanisms.
o Added mention of 'code' attribute on error element.
o Clarified several sentences in the dialback narrative.
o Clarified use of stringprep profiles and added reference to RFC
3490.
o Added security consideration regarding lack of SASL channel
binding to TLS per discussion at IETF 58 meeting.
o Adjusted formatting to conform to RFC Editor requirements.
E.2 Changes from draft-ietf-xmpp-core-18
o Added the 'xml:lang' attribute to the root <stream/> element per
previous consensus and list discussion.
o Changed "jabber-server" and "jabber-client" service names to
"xmpp-server" and "xmpp-client".
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o Added the <gone/>, <not-acceptable/>, and <redirect/> stanza
errors.
o Changed dataype of <see-other-host/> stream error and of <gone/>
and <redirect/> stanza errors to xs:string so that these elements
may contain programmatic information.
o Removed <invalid-realm/> and <bad-protocol/> SASL errors.
o Removed references to RFC 952 and RFC 1123 (domain name format is
handled by reference to Nameprep).
o Changed address record resolution text so that it is not specific
to IPv4.
o Clarified text in appendices regarding scope of Nodeprep and
Resourceprep.
o Removed requirement that receiving entity terminate the TCP
connection upon receiving an <abort/> element from or sending a
<failure/> element to the initiating entity during SASL
negotiation.
o Removed recommendation that TLS and SASL security layer should not
both be used simultaneously.
o Added subsection to Security Considerations regarding use of
base64 in SASL.
o Specified rules regarding inclusion of username in SASL
negotiation.
o Adjusted content related to SASL authorization identities, since
the previous text did not track SASL.
o Added section on resource binding to compensate for changes to
SASL authorization identity text.
o Specified ABNF for JIDs.
o Checked all references.
o Completed a thorough proofreading and consistency check of the
entire text.
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E.3 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.4 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.
E.5 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.
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E.6 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.7 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.8 Changes from draft-ietf-xmpp-core-12
o Moved server dialback to a separate section; clarified its
security characteristics and its role in the protocol.
o Adjusted error handling syntax and semantics per list discussion.
o Further clarified length of node identifiers and total length of
JIDs.
o Documented message type='normal'.
o Corrected several small errors in the TLS and SASL sections.
o Corrected several errors in the schemas.
E.9 Changes from draft-ietf-xmpp-core-11
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o Corrected several small errors in the TLS and SASL sections.
o Made small editorial changes and fixed several schema errors.
E.10 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.11 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.12 Changes from draft-ietf-xmpp-core-08
o Incorporated list discussion regarding addressing, SASL, TLS, TCP,
dialback, namespaces, extensibility, and the meaning of 'ignore'
for routers and recipients.
o Specified dialback error conditions.
o Made small editorial changes to address RFC Editor requirements.
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E.13 Changes from draft-ietf-xmpp-core-07
o Made several small editorial changes.
E.14 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.
o Removed note to RFC Editor regarding provisional namespace names.
E.15 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.16 Changes from draft-ietf-xmpp-core-04
o Added server-to-server examples for TLS and SASL.
o Changed error syntax, rules, and examples based on list
discussion.
o Added schemas for the TLS, stream error, and stanza error
namespaces.
o Added note to RFC Editor regarding provisional namespace names.
o Made numerous small editorial changes and clarified text
throughout.
E.17 Changes from draft-ietf-xmpp-core-03
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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.18 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.19 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 Use of SASL regarding 'version' attribute.
o Made numerous small editorial changes.
E.20 Changes from draft-ietf-xmpp-core-00
o Added information about TLS from list discussion.
o Clarified meaning of "ignore" based on list discussion.
o Clarified information about Universal Character Set data and
character encodings.
o Provided base64-decoded information for examples.
o Fixed several errors in the schemas.
o Made numerous small editorial fixes.
E.21 Changes from draft-miller-xmpp-core-02
o Brought Use of SASL section into line with discussion on list and
at IETF 55 meeting.
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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.
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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Saint-Andre (ed.) Expires May 20, 2004 [Page 92]
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Saint-Andre (ed.) Expires May 20, 2004 [Page 93]
