MMUSIC O. Levin, Ed.
Internet-Draft Microsoft Corporation
Expires: August 15, 2004 R. Even
Polycom
P. Hagendorf
RADVISION
February 15, 2004
XML Schema for Media Control
draft-levin-mmusic-xml-schema-media-control-03
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 15, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines an XML Schema for Media Control in a tightly
controlled environment. The current version includes commands for
managing of video streams only. Implementation of this schema for
interactive video applications in SIP environments significantly
improves user experience.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. The Video Control Commands . . . . . . . . . . . . . . . . . . 6
5. The Schema Definition . . . . . . . . . . . . . . . . . . . . 7
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. The Schema Applications . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. URN for XML schema . . . . . . . . . . . . . . . . . . . . . . 15
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16
12. Changes since -02 . . . . . . . . . . . . . . . . . . . . . . 17
Normative References . . . . . . . . . . . . . . . . . . . . . 18
Informative References . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . 20
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1. Conventions
The 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 [1].
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2. Introduction
This document defines an XML Schema for Media Control in a tightly
controlled environment. The current version includes commands for
managing of video streams only. Implementation of this schema for
interactive video applications in SIP [3] environments significantly
improves user experience. Both end users and conferencing servers
SHOULD implement this mechanism.
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3. Background
SIP typically uses RTP [6] for transferring of real time media.
RTP is augmented by a control protocol (RTCP) to allow monitoring of
the data delivery in a manner scalable to large multicast networks.
The RTCP feedback mechanism [10] has been introduced in order to
improve basic RTCP feedback time in case of loss conditions across
different coding schemes. (Previously, a subset of this functionality
was defined for H.261 [2] only).
The mentioned above techniques address signaling of loss conditions
and the recommended recovery steps. Currently, there is no standard
way to express required operations on media streams as a result of
application logic rather than a result of loss conditions.
Of course, each command MUST be validated by the remote entity
against current media capacity and network conditions before being
executed.
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4. The Video Control Commands
Output of a video CODEC is a frame. The frame can carry complete (in
time) information about a picture or about a picture segment. These
frames are known as "Intra" frames. In order to save bandwidth, other
frames can carry only changes relative to previously sent frames.
Frames carrying relative information are known as "Inter" frames.
Some CODECs (such as H.261 and H.263), in addition to a "full"
picture, have a notion of picture slices: MB (Macro Block) and GOB
(Group Of Blocks).
Based on application logic (such as need to present a new video
source), the application needs to have an ability to explicitly
request from a remote encoder the complete (in-time) information
about a "full" picture or a specific slice of the picture.
Additionally, in case the encoder is aware of upcoming changes in the
transmitted stream (that would result in synchronization lost by the
decoder), the encoder needs to be able to request the decoder to
freeze the picture, i.e. to stop presenting the changes until a new
stable image is encoded and transmitted.
In order to meet the presented requirements, four video primitives
are defined by this document.
The following commands are sent to the remote encoder:
o Video Picture Fast Update
o Video GOB Fast Update (first_GOB, no_of_GOBs)
o Video MB Fast Update (first_GOB, first_MB, no_of_MBs)
The following command is sent to the remote decoder:
o Video Picture Freeze
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5. The Schema Definition
<?xml version="1.0" encoding="utf-8" ?>
<xsd:schema id="TightMediaControl" targetNamespace="urn:ietf:params:xml:ns:media_control" elementFormDefault="qualified" xmlns=" urn:ietf:params:xml:ns:media_control" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:element name="media_control">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="vc_primitive" type="vc_primitive" minOccurs="0" maxOccurs="unbounded" />
<xsd:element name="general_error" type="xs:string" minOccurs="0" maxOccurs="unbounded" />
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<!-- Video control primitive. Each primitive is directed either to a remote encode or to a remote decoder. -->
<xsd:complexType name="vc_primitive">
<xsd:sequence>
<xsd:choice>
<xsd:element name="to_encoder" type="to_encoder" />
<xsd:element name="to_decoder" type="to_decoder" />
</xsd:choice>
<xsd:element name="stream_id" type="xsd:string" minOccurs="0" maxOccurs="unbounded" />
</xsd:sequence>
<xsd:attribute name="type" type="primitive_type" use="optional" default="cmd" />
<xsd:attribute name="stream" type="stream_type" use="optional" default="all" />
</xsd:complexType>
<xsd:simpleType name="primitive_type">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="cmd" />
<xsd:enumeration value="ind" />
<xsd:enumeration value="req" />
<xsd:enumeration value="rsp" />
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="stream_type">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="mid" />
<xsd:enumeration value="all" />
</xsd:restriction>
</xsd:simpleType>
<!-- Decoder commands:
Currently only Picture Freeze is defined.
-->
<xsd:complexType name="to_decoder">
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<xsd:sequence>
<xsd:element name="picture_freeze" />
</xsd:sequence>
</xsd:complexType>
<!-- Encoder Commands:
Picture Fast Update
GOB Fast Update
MB Fast Update
-->
<xsd:complexType name="to_encoder">
<xsd:choice>
<xsd:element name="picture_fast_update" />
<xsd:element name="gob_fast_update">
<xsd:complexType>
<xsd:attribute name="first_gob" type="xsd:nonNegativeInteger" use="optional" default="0" />
<xsd:attribute name="no_of_gobs" type="xsd:nonNegativeInteger" use="optional" default="1" />
</xsd:complexType>
</xsd:element>
<xsd:element name="mb_fast_update">
<xsd:complexType>
<xsd:attribute name="first_gob" type="xsd:nonNegativeInteger" use="optional" default="0" />
<xsd:attribute name="first_mb" type="xsd:nonNegativeInteger" use="optional" default="1" />
<xsd:attribute name="no_of_mbs" type="xsd:nonNegativeInteger" use="optional" default="1" />
</xsd:complexType>
</xsd:element>
</xsd:choice>
</xsd:complexType>
</xsd:schema>
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6. Error Handling
Currently, only a single general error primitive is defined. It MAY
be used for indicating errors in free text format. The general error
primitive MAY report problems regarding XML document parsing,
inadequate level of media control support, inability to perform the
requested action, etc.
The general error primitive MUST NOT be used for indication of errors
other than related to media control parsing or to resultant
execution. The general error primitive MUST NOT be sent back as a
result of getting an error primitive.
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7. Examples
In the following example the "Freeze" and the full picture "Fast
Update" commands are issued towards the remote video decoder(s) and
remote video encoder(s) respectively for all video streams.
<?xml version="1.0" encoding="utf-8" ?>
<media_control>
<vc_primitive>
<to_decoder>
<picture_freeze>
</picture_freeze>
</to_decoder>
</vc_primitive>
<vc_primitive>
<to_encoder>
<picture_fast_update>
</picture_fast_update>
</to_encoder>
</vc_primitive>
</media_control>
If an error occurs during the parsing of the XML document, the
following XML document would be sent back to the originator of the
original Media Control document.
<?xml version="1.0" encoding="utf-8" ?>
<media_control>
<general_error>
Parsing error: The original XML segment is:...
</general_error>
</media_control>
The following example demonstrates the full format of the defined
schema. The "Freeze" command is issued towards the remote video
decoder(s) to be applied to streams with "mid" [5] values "1" and
"2". The GOB "Fast Update" command is issued towards the remote
video encoder to be applied to a video stream with "mid" value "3".
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<?xml version="1.0" encoding="utf-8" ?>
<media_control>
<vc_primitive stream=mid>
<to_decoder >
<picture_freeze>
</picture_freeze>
</to_decoder>
<stream_id>1</stream_id>
<stream_id>2</stream_id>
</vc_primitive>
<vc_primitive stream="mid">
<to_encoder>
<mb_fast_update first_gob="2" first_mb="3" no_of_mbs="4"> </mb_fast_update>
</to_encoder>
<stream_id>3</stream_id>
</vc_primitive>
</media_control>
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8. The Schema Applications
The defined XML document is conveyed using SIP INFO method [4] with
the "Content-Type" set to "application/media_control+xml". This
approach benefits from the SIP built-in reliability.
The authors plan registering the defined schema with IANA according
to the guidelines specified in [9] and issuing a separate SIPPING
usage document(s). The document(s) will describe procedures for
conveying an XML document defined according to the Schema by means of
SIP INFO and SIP NOTIFY.
The authors hope that the XML schema, documented in this document,
will provide a base for a standard Tight Media Control protocol
definition within the IETF. It is expected that in future SIP will
define standard means for running this protocol as a part of SIP
architecture.
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9. IANA Considerations
Application/media_control+xml MIME type
MIME Media Type Name: application
MIME subtype name: media_control+xml
Required parameters: None
Optional parameters: charset
See RFC 3023 [7] for a discussion of the charset parameter on
XML-derived MIME types. Since this MIME type is used exclusively in
SIP, the use of UTF-8 encoding is strongly encouraged.
Encoding considerations: 8-bit text
Security considerations: Security considerations specific to uses of
this this MIME type are discussed in RFC xxxx [[Note to RFC
editor: replace xxxx with the RFC number of this document when
published]]. RFC 1874 [8] and RFC 3023 [7] discuss security
issues common to all uses of XML.
Interoperability considerations: The use of this MIME body is
intended to be generally interoperable. No unique considerations
have been identified.
Published specification: RFC xxxx [[Note to RFC editor: replace xxxx
with the RFC number of this document when published]]
Applications which use this media type: This media type is used to
convey information regarding media control commands and responses
between SIP endpoints particularly for allowing a Video Fast
Update intra-frame request.
Additional information:
Magic Number(s): None.
File Extension(s): None.
Macintosh File Type Code(s): None.
Object Identifier(s) or OID(s): None.
Intended usage: Limited Use
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Other Information/General Comment: None.
Person to contact for further information:
Name: Orit Levin
E-Mail: oritl@microsoft.com
Author/Change Controller:
The specification of this MIME type is an individual submission to
the MMUSIC working group of the IETF. Change control of this
specification is under control of the IETF WG.
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10. URN for XML schema
URI: urn:ietf:params:xml:ns:media_control
Description:
This is the XML namespace URI for XML elements defined by [RFCXXXX]
to describe information exchanged between SIP endpoints for media
control. It is used in the application/media_control+xml body type.
Registrant Contact:
Name: Orit Levin
E-Mail: oritl@microsoft.com
Author/Change Controller:
The specification of this MIME type is an individual submission to
the MMUSIC working group of the IETF. Change control of this
specification is under control of the IETF WG.
XML:
BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type"
content="text/html;charset=utf-8"/>
<title>Namespace for Tight Media Control</title>
</head>
<body>
<h1>Namespace for Tight Media Control</h1>
<h2>application/media_control+xml</h2>
<p>See <a href="[[[URL of published RFC]]]">RFCXXXX</a>.</p>
</body>
</html>
END
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11. Security Considerations
This document doesnt introduce new security requirements beyond
covered in [4].
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12. Changes since -02
o This version contains editorial changes only.
o The XML schema is identical to version -02.
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Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Turletti, T., "RTP Payload Format for H.261 Video Streams", RFC
2032, October 1996.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[4] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.
[5] Camarillo, G., Eriksson, G., Holler, J. and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
[6] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
[7] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC
3023, January 2001.
[8] Levinson, E., "SGML Media Types", RFC 1874, December 1995.
[9] Mealling, M., "The IETF XML Registry",
draft-mealling-iana-xmlns-registry-05 (work in progress), June
2003.
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Informative References
[10] Ott, J. and S. Wenger, "Extended RTP Profile for RTCP-based
Feedback(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-08 (work in
progress), February 2004.
Authors' Addresses
Orit Levin (editor)
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: oritl@microsoft.com
Roni Even
Polycom
94 Derech Em Hamoshavot
Petach Tikva, 49130
Israel
EMail: roni.even@polycom.co.il
Pierre Hagendorf
RADVISION
24, Raul Wallenberg St.
Tel-Aviv, 69719
Israel
EMail: pierre@radvision.com
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