XMPP Working Group L. Stout, Ed.
Internet-Draft &yet
Intended status: Standards Track J. Moffitt
Expires: February 12, 2015 Mozilla
E. Cestari
cstar industries
August 11, 2014
An XMPP Sub-protocol for WebSocket
draft-ietf-xmpp-websocket-09
Abstract
This document defines a binding for the XMPP protocol over a
WebSocket transport layer. A WebSocket binding for XMPP provides
higher performance than the current HTTP binding for XMPP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on February 12, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. XMPP Sub-Protocol . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. WebSocket Messages . . . . . . . . . . . . . . . . . . . 4
3.3. XMPP Framing . . . . . . . . . . . . . . . . . . . . . . 5
3.3.1. Framed XML Stream . . . . . . . . . . . . . . . . . . 5
3.3.2. Framed Stream Namespace . . . . . . . . . . . . . . . 5
3.3.3. Stream Frames . . . . . . . . . . . . . . . . . . . . 5
3.4. Stream Initiation . . . . . . . . . . . . . . . . . . . . 6
3.5. Stream Errors . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Closing the Connection . . . . . . . . . . . . . . . . . 7
3.6.1. see-other-uri . . . . . . . . . . . . . . . . . . . . 8
3.7. Stream Restarts . . . . . . . . . . . . . . . . . . . . . 9
3.8. Pings and Keepalives . . . . . . . . . . . . . . . . . . 9
3.9. Use of TLS . . . . . . . . . . . . . . . . . . . . . . . 9
3.10. Stream Management . . . . . . . . . . . . . . . . . . . . 10
4. Discovering the WebSocket Connection Method . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5.1. WebSocket Subprotocol Name . . . . . . . . . . . . . . . 11
5.2. URN Sub-Namespace . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Appendix B. XML Schema . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
To date, applications using the Extensible Messaging and Presence
Protocol (XMPP) (see [RFC6120] and [RFC6121]) on the Web have made
use of BOSH (see [XEP-0124] and [XEP-0206]), an XMPP binding to HTTP.
BOSH is based on the HTTP long polling technique, and it suffers from
high transport overhead compared to XMPP's native binding to TCP. In
addition, there are a number of other known issues with long polling
[RFC6202], which have an impact on BOSH-based systems.
It would be much better in most circumstances to avoid tunneling XMPP
over HTTP long polled connections and instead use the XMPP protocol
directly. However, the APIs and sandbox that browsers have provided
do not allow this. The WebSocket protocol [RFC6455] exists to solve
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these kinds of problems and is a bidirectional protocol that provides
a simple message-based framing layer, allowing for more robust and
efficient communication in web applications.
The WebSocket protocol enables two-way communication between a client
and a server, effectively emulating TCP at the application layer and
therefore overcoming many of the problems with existing long-polling
techniques for bidirectional HTTP. This document defines a WebSocket
sub-protocol for XMPP.
The WebSocket binding for XMPP is designed for use by browser-based
applications (e.g., XMPP clients written in JavaScript). These
applications typically are used to access the same information and
communication opportunities (e.g., the same XMPP "roster" of
contacts) as clients that access connect to an XMPP server over the
TCP binding defined in [RFC6120]. Although the only essential
difference is the underlying transport binding, relevant implications
(e.g., framing methods and discovery processes) are highlighted in
this specification.
2. Terminology
The basic unit of framing in the WebSocket protocol is called a
message. In XMPP, the basic unit is the stanza, which is a subset of
the first-level children of each document in an XMPP stream (see
Section 9 of [RFC6120]). XMPP also has a concept of messages, which
are stanzas with a top-level element of <message/>. In this
document, the word "message" will mean a WebSocket message, not an
XMPP message stanza, unless otherwise noted.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. XMPP Sub-Protocol
3.1. Handshake
The XMPP sub-protocol is used to transport XMPP over a WebSocket
connection. The client and server agree to this protocol during the
WebSocket handshake (see Section 1.3 of [RFC6455]).
During the WebSocket handshake, the client MUST include the value
'xmpp' in the list of protocols for the 'Sec-WebSocket-Protocol'
header. The reply from the server MUST also contain 'xmpp' in its
own 'Sec-WebSocket-Protocol' header in order for an XMPP sub-protocol
connection to be established.
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If a client receives a handshake response that does not include
'xmpp' in the 'Sec-WebSocket-Protocol' header, then a XMPP sub-
protocol WebSocket connection was not established and the client MUST
close the WebSocket connection.
Once the handshake has successfully completed, WebSocket messages
sent or received MUST conform to the protocol defined in the rest of
this document.
The following is an example of a WebSocket handshake, followed by
opening an XMPP stream:
C: GET /xmpp-websocket HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Origin: http://example.com
...
Sec-WebSocket-Protocol: xmpp
Sec-WebSocket-Version: 13
S: HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
...
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: xmpp
[WebSocket connection established]
C: <open xmlns="urn:ietf:params:xml:ns:xmpp-framing"
to="example.com"
version="1.0" />
S: <open xmlns="urn:ietf:params:xml:ns:xmpp-framing"
from="example.com"
id="++TR84Sm6A3hnt3Q065SnAbbk3Y="
xml:lang="en"
version="1.0" />
3.2. WebSocket Messages
Data frame messages in the XMPP sub-protocol MUST be of the text type
and contain UTF-8 encoded data.
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3.3. XMPP Framing
The framing method for the binding of XMPP to WebSocket differs from
the framing method for the TCP binding as defined in [RFC6120]; in
particular, the WebSocket binding adopts the message framing provided
by WebSocket to delineate the stream open and close headers, stanzas,
and other top-level stream elements.
3.3.1. Framed XML Stream
The start of a framed XML stream is marked by the use of an opening
"stream header" which is an <open/> element with the appropriate
attributes and namespace declarations (see Section 3.3.2). The
attributes of the <open/> element are the same as those of the
<stream/> element defined defined for the 'http://etherx.jabber.org/
streams' namespace in [RFC6120] and with the same semantics and
restrictions.
The end of a framed XML stream is denoted by the closing "stream
header" which is a <close/> element with its associated attributes
and namespace declarations (see Section 3.3.2).
The introduction of the <open/> and <close/> elements is motivated by
the parsable XML document framing restriction in Section 3.3.3. As a
consequence, note that a framed XML stream does not provided a
wrapping <stream:stream/> element encompassing the entirety of the
XML stream, as in [RFC6120].
3.3.2. Framed Stream Namespace
The XML stream "headers" (the <open/> and <close/> elements) MUST be
qualified by the namespace 'urn:ietf:params:xml:ns:xmpp-framing' (the
"framed stream namespace"). If this rule is violated, the entity
that receives the offending stream header MUST close the stream with
an error, which MUST be <invalid-namespace> (see Section 4.9.3.10 of
[RFC6120]).
3.3.3. Stream Frames
The individual frames of a framed XML stream have a one-to-one
correspondence with WebSocket messages, and MUST be parsable as
standalone XML documents, complete with all relevant namespace and
language declarations. The inclusion of XML declarations, however,
is NOT RECOMMENDED, as WebSocket messages are already mandated to be
UTF-8 encoded. Inclusions of declarations would only add a constant
size overhead to each message.
The first character of each frame MUST be a '<' character.
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Every XMPP stanza or other XML element (including the stream open and
close headers) sent directly over the XML stream MUST be sent in its
own frame.
Example of a WebSocket message that contains an independently
parsable XML document:
<message xmlns="jabber:client" xml:lang="en">
<body>Every WebSocket message is parsable by itself.</body>
</message>
Note that for stream features and errors, there is no parent context
element providing the "stream" namespace prefix as in [RFC6120], and
thus the stream prefix MUST be declared or use an unprefixed form:
<stream:features xmlns:stream="http://etherx.jabber.org/streams">
<bind xmlns="urn:ietf:params:xml:ns:xmpp-bind"/>
</stream:features>
-- OR --
<error xmlns="http://etherx.jabber.org/streams">
<host-unknown xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</error>
3.4. Stream Initiation
The first message sent after the WebSocket opening handshake MUST be
from the initiating entity, and MUST be an <open/> element qualified
by the 'urn:ietf:params:xml:ns:xmpp-framing' namespace and with the
same attributes mandated for the <stream> opening tag as described in
Section 4.7 of [RFC6120].
The receiving entity MUST respond with either an <open /> element
(whose attributes match those described in Section 4.7 of [RFC6120])
or a <close /> element (see Section 3.6.1).
An example of a successful stream initiation exchange:
C: <open xmlns="urn:ietf:params:xml:ns:xmpp-framing"
to="example.com"
version="1.0" />
S: <open xmlns="urn:ietf:params:xml:ns:xmpp-framing"
from="example.com"
id="++TR84Sm6A3hnt3Q065SnAbbk3Y="
xml:lang="en"
version="1.0" />
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Clients MUST NOT multiplex XMPP streams over the same WebSocket.
3.5. Stream Errors
Stream level errors in XMPP are fatal. Should such an error occur,
the server MUST send the stream error as a complete element in a
message to the client.
If the error occurs during the opening of a stream, the server MUST
send the initial open element response, followed by the stream level
error in a second WebSocket message frame. The server MUST then
close the connection as specified in Section 3.6.
3.6. Closing the Connection
The closing process for the XMPP sub-protocol mirrors that of the
XMPP TCP binding as defined in Section 4.4 of [RFC6120], except that
a <close/> element is used instead of the ending </stream:stream>
tag.
Either the server or the client may close the connection at any time.
Before closing the connection, the closing party is expected to first
close the XMPP stream (if one has been opened) by sending a message
with the <close/> element, qualified by the "urn:ietf:params:xml:ns
:xmpp-framing" namespace. The stream is considered closed when a
corresponding <close/> element is received from the other party, and
the XMPP session is ended.
To then close the WebSocket connection, the closing party MUST
initiate the WebSocket closing handshake (see Section 7.1.2 of
[RFC6455]).
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An example of ending an XMPP over WebSocket session by first closing
the XMPP stream layer and then the WebSocket connection layer:
Client (XMPP WSS) Server
| | | |
| | <close xmlns="urn:ietf:params:xml:ns:xmpp-framing /> | |
| |------------------------------------------------------------>| |
| | <close xmlns="urn:ietf:params:xml:ns:xmpp-framing" /> | |
| |<------------------------------------------------------------| |
| | | |
| | (XMPP Stream Closed) | |
| +-------------------------------------------------------------+ |
| |
| WS CLOSE FRAME |
|------------------------------------------------------------------>|
| WS CLOSE FRAME |
|<------------------------------------------------------------------|
| |
| (Connection Closed) |
+-------------------------------------------------------------------+
If the WebSocket connection is closed or broken without the XMPP
stream having been closed first, then the XMPP stream is considered
implicitly closed and the XMPP session ended; however, if the use of
stream management resumption was negotiated (see [XEP-0198]), the
server SHOULD consider the XMPP session still alive for a period of
time based on server policy as specified in [XEP-0198].
3.6.1. see-other-uri
If the server wishes at any point to instruct the client to move to a
different WebSocket endpoint (e.g., for load balancing purposes),
then a <close/> element is sent with the 'see-other-uri' attribute
set to the URI of the new connection endpoint (which MAY be for a
different transport method, such as BOSH (see [XEP-0124] and
[XEP-0206])).
Clients MUST NOT accept suggested endpoints with a lower security
context (e.g., moving from a 'wss://' endpoint to a 'ws://' or 'http:
//' endpoint).
An example of the server closing a stream and instructing the client
to connect at a different WebSocket endpoint:
S: <close xmlns="urn:ietf:params:xml:ns:xmpp-framing"
see-other-uri="wss://otherendpoint.example/xmpp-bind" />
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3.7. Stream Restarts
Whenever a stream restart is mandated (see Section 4.3.3 of
[RFC6120]), both the server and client streams are implicitly closed
and new streams MUST be opened, using the same process as in
Section 3.4.
The client MUST send a new stream <open/> element and MUST NOT send a
closing <close/> element.
An example of restarting the stream after successful SASL
negotiation:
S: <success xmlns="urn:ietf:params:xml:ns:xmpp-sasl" />
[Streams implicitly closed]
C: <open xmlns="urn:ietf:params:xml:ns:xmpp-framing"
to="example.com"
version="1.0" />
3.8. Pings and Keepalives
Traditionally, XMPP servers and clients often send "whitespace
keepalives" (see Section 4.6.1 of [RFC6120]) between stanzas to
maintain an XML stream. However, for the XMPP sub-protocol each
message is required to start with a '<' character, and, as such,
whitespace keepalives MUST NOT be used.
As alternatives, the XMPP Ping extension [XEP-0199] and the XMPP
Stream Management extension [XEP-0198] provide pinging mechanisms.
Either of these extensions (or both) MAY be used to determine the
state of the connection.
Clients and servers MAY also use WebSocket ping control frames for
this purpose, but note that some environments, such as browsers, do
not provide access for generating or monitoring ping control frames.
3.9. Use of TLS
TLS cannot be used at the XMPP sub-protocol layer because the sub-
protocol does not allow for raw binary data to be sent. Instead,
when TLS is used, it MUST be enabled at the WebSocket layer using
secure WebSocket connections via the 'wss' URI scheme. (See
Section 10.6 of [RFC6455].)
Because TLS is to be provided outside of the XMPP sub-protocol layer,
a server MUST NOT advertise TLS as a stream feature (see Section 4.6
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of [RFC6120]) when using the XMPP sub-protocol. Likewise, a client
MUST ignore any advertised TLS stream feature when using the XMPP
sub-protocol.
3.10. Stream Management
In order to alleviate the problems of temporary disconnections, the
client MAY use the XMPP Stream Management extension [XEP-0198] to
confirm when stanzas have been received by the server.
In particular, the client MAY use session resumption in [XEP-0198] to
recreate the same stream session state after a temporary network
unavailability or after navigating to a new URL in a browser.
4. Discovering the WebSocket Connection Method
Section 3 of [RFC6120] defines a procedure for connecting to an XMPP
server, including ways to discover the TCP/IP address and port of the
server using Domain Name System service (DNS SRV) records [RFC2782].
When using the WebSocket binding as specified in this document
(instead of the TCP binding as specified in [RFC6120]), a client
needs an alternative way to discover information about the server's
connection methods, since web browsers and other WebSocket-capable
software applications typically cannot obtain such information from
the DNS.
The alternative lookup process uses Web Host Metadata [RFC6415] and
Web Linking [RFC5988], where the link relation type is "urn:xmpp:alt-
connections:websocket" as described in Discovering Alternate XMPP
Connection Methods [XEP-0156]. Conceptually, the host-meta lookup
process used for the WebSocket binding is analogous to the DNS SRV
lookup process used for the TCP binding. The process is as follows.
1. Send a request over secure HTTP to the path "/.well-known/host-
meta" at an HTTP origin [RFC6454] that matches the XMPP service
domain (e.g., a URL of "https://im.example.org/.well-known/host-
meta" if the XMPP service domain is "im.example.org").
2. Retrieve a host-meta document specifying a link relation type of
"urn:xmpp:alt-connections:websocket", such as:
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>
<Link rel="urn:xmpp:alt-connections:websocket"
href="wss://im.example.org:443/ws" />
</XRD>
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Servers MAY expose discovery information using host-meta documents,
and clients MAY use such information to determine the WebSocket
endpoint for a server.
In cases where the XMPP service domain does not match the web origin
of the WebSocket endpoint, the Web-host metadata MAY be used to
establish trust between the XMPP server domain and the WebSocket
endpoint as long as the host-meta request and response occurred over
secure HTTP; this is especially relevant in multi-tenant situations
where the same WebSocket endpoint is serving multiple XMPP domains
(e.g., the XMPP service domains for both "example.com" and
"im.example.org" might be serviced by the same WebSocket endpoint at
"hosting.example.net"). See Section 6 for related discussion.
5. IANA Considerations
5.1. WebSocket Subprotocol Name
This specification requests IANA to register the WebSocket XMPP sub-
protocol under the "WebSocket Subprotocol Name" Registry with the
following data:
Subprotocol Identifier: xmpp
Subprotocol Common Name: WebSocket Transport for the Extensible
Messaging and Presence Protocol (XMPP)
Subprotocol Definition: this document
5.2. URN Sub-Namespace
A URN sub-namespace for framing of Extensible Messaging and Presence
Protocol (XMPP) streams is defined as follows.
URI: urn:ietf:params:xml:ns:xmpp-framing
Specification: this document
Description: This is the XML namespace name for framing of
Extensible Messaging and Presence Protocol (XMPP) streams as
defined by RFC XXXX.
Registrant Contact: IESG <iesg@ietf.org>
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6. Security Considerations
Since application level TLS cannot be used (see Section 3.9),
applications need to protect the privacy of XMPP traffic at the
WebSocket or other appropriate layer.
Browser-based applications are not able to inspect and verify, at the
application layer, the certificate used for the WebSocket connection
to ensure that it corresponds to the domain specified as the 'to'
address of the XMPP stream. There are two cases:
1. If the XMPP service domain matches the origin for the WebSocket
connection, the relevant check is already performed by the
browser. For example, the XMPP service domain might be
"foo.example" and the WebSocket endpoint discovered for the link
relation type of "urn:xmpp:alt-connections:websocket" might be
"wss://foo.example/websocket". As long as the certificate
provided over WebSocket or HTTPS is verified according to the
rules defined for secure HTTP [RFC2818], then the browser will
report the successful establishment of a secure connection to the
application. (However, as noted, the application is still not
able to independently inspect and verify the certificate, and
needs to trust the browser; this is a limitation of existing
browser technologies, and thus cannot be worked around by
WebSocket applications.)
2. In situations where the domain of the XMPP server does not match
the web origin of the WebSocket endpoint (such as multi-tenant
hosting situations, the host-meta process described under
Section 4) MAY be used to delegate trust from the XMPP server
domain to the WebSocket origin, as long as the host-meta request
and response occurred over secure HTTP (with appropriate
certificate verification as defined in [RFC2818]).
When presented with a new WebSocket endpoint via the 'see-other-uri'
attribute of a <close/> element, clients MUST NOT accept the
suggestion if the security context of the new endpoint is lower than
the current one in order to prevent downgrade attacks from a 'wss://'
endpoint to 'ws://'.
The Security Considerations for both WebSocket (see Section 10 of
[RFC6455]) and XMPP (see Section 13 of [RFC6120]) apply to the
WebSocket XMPP sub-protocol.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6415] Hammer-Lahav, E. and B. Cook, "Web Host Metadata", RFC
6415, October 2011.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
6455, December 2011.
7.2. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC6121] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence", RFC
6121, March 2011.
[RFC6202] Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
"Known Issues and Best Practices for the Use of Long
Polling and Streaming in Bidirectional HTTP", RFC 6202,
April 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
[XEP-0124]
Paterson, I., Smith, D., Saint-Andre, P., Moffitt, J., and
L. Stout, "Bidirectional-streams Over Synchronous HTTP
(BOSH)", XSF XEP 0124, November 2013.
[XEP-0156]
Hildebrand, J., Saint-Andre, P., and L. Stout,
"Discovering Alternative XMPP Connection Methods", XSF XEP
0156, January 2014.
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[XEP-0198]
Karneges, J., Saint-Andre, P., Hildebrand, J., Forno, F.,
Cridland, D., and M. Wild, "Stream Management", XSF XEP
0198, June 2011.
[XEP-0199]
Saint-Andre, P., "XMPP Ping", XSF XEP 0199, June 2009.
[XEP-0206]
Paterson, I., Saint-Andre, P., and L. Stout, "XMPP Over
BOSH", XSF XEP 0206, November 2013.
[XML-SCHEMA]
Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech,
"XML Schema Part 1: Structures Second Edition", World Wide
Web Consortium Recommendation REC-xmlschema-1-20041028,
October 2004,
<http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.
Appendix A. Acknowledgements
The authors wish to thank the following individuals for their
feedback: Andreas Guth, Bjoern Hoerhmann, Dave Cridland, Florian
Zeitz, Kurt Zeilenga, Matt Miller, Matthew Wild, Paul Aurich, Sergey
Dobrov, Waqas Hussain.
Dan Romascanu reviewed the document on behalf of the General Area
Review Team.
During IESG review, Barry Leiba Benoit Claise, Dan Romasanu, Jari
Arkko, Juergen Schoenwaelder, Spencer Dawkins, Stephen Farrell, Ted
Lemon, Kathleen Moriarty, Pete Resnick caught several issues that
needed to be addressed.
The authors gratefully acknowledge the assistance of Peter Saint-
Andre as document shepherd, Ben Campbell and Joe Hildebrand as the
working group chairs, and Richard Barnes as the sponsoring Area
Director.
Appendix B. XML Schema
The following schema formally defines the 'urn:ietf:params:xml:ns
:xmpp-framing' namespace used in this document, in conformance with
W3C XML Schema [XML-SCHEMA]. Because validation of XML streams and
stanzas is optional, this schema is not normative and is provided for
descriptive purposes only.
<?xml version='1.0' encoding='UTF-8'?>
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<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-framing'
xmlns='urn:ietf:params:xml:ns:xmpp-framing'
elementFormDefault='unqualified'>
<xs:element name='open'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='empty'>
<xs:attribute name='from' type='xs:string'
use='optional'/>
<xs:attribute name='id' type='xs:string'
use='optional'/>
<xs:attribute name='to' type='xs:string'
use='optional'/>
<xs:attribute name='version' type='xs:decimal'
use='optional'/>
<xs:attribute ref='xml:lang'
use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='close'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='empty'>
<xs:attribute name='from' type='xs:string'
use='optional'/>
<xs:attribute name='id' type='xs:string'
use='optional'/>
<xs:attribute name='see-other-uri' type='xs:anyURI'
use='optional'/>
<xs:attribute name='to' type='xs:string'
use='optional'/>
<xs:attribute name='version' type='xs:decimal'
use='optional'/>
<xs:attribute ref='xml:lang'
use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
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<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
Authors' Addresses
Lance Stout (editor)
&yet
Email: lance@andyet.net
Jack Moffitt
Mozilla
Email: jack@metajack.im
Eric Cestari
cstar industries
Email: eric@cstar.io
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