MMUSIC O. Levin, Ed.
Internet-Draft Microsoft Corporation
Expires: September 4, 2006 R. Even
Polycom
P. Hagendorf
RADVISION
March 03, 2006
XML Schema for Media Control
draft-levin-mmusic-xml-media-control-04
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines an XML Schema for Media Control in a tightly
controlled environment for managing of video streams only.
Implementation of this mechanism for interactive video applications
in SIP environments significantly improves user experience.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. The Video Control Commands . . . . . . . . . . . . . . . . . . 3
5. The Schema Definition . . . . . . . . . . . . . . . . . . . . 4
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. The Fast Update command for the full picture . . . . . . . 7
7.2. Reporting an error . . . . . . . . . . . . . . . . . . . . 8
7.3. Freeze and Fast Update commands for the full picture . . . 8
7.4. Advanced XML schema usage . . . . . . . . . . . . . . . . 8
8. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. URN for XML schema . . . . . . . . . . . . . . . . . . . . . . 10
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. Changes History . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Changes since -03 . . . . . . . . . . . . . . . . . . . . 12
12.2. Changes since -02 . . . . . . . . . . . . . . . . . . . . 12
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
13.1. Normative References . . . . . . . . . . . . . . . . . . . 12
13.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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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 [3].
2. Introduction
This document defines an XML Schema for Media Control in a tightly
controlled environment for managing of video streams only.
Implementation of this schema for interactive video applications in
SIP [6] environments significantly improves user experience. Both
end users and conferencing servers need to implement this mechanism.
3. Background
SIP typically uses RTP [8] 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). Theses techniques
address signaling of loss conditions and the recommended recovery
steps.
Just recently an extension to the feedback mechanism has been
proposed [11] to express control operations on media streams as a
result of application logic rather than a result of loss conditions.
Note that in the decomposed systems the implementation of the new
mechanism will require proprietary communications between the
applications/call control components and the media components.
This document describes the technology being deployed over the
Internet for over the last three years and being used across real-
time interactive applications from different vendors in interoperable
manner.
Of course, each command MUST be validated by the remote entity
against current media capacity and network conditions before being
executed.
4. The Video Control Commands
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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
Note that out of all the defined primitives only the "Video Picture
Fast Update" is currently being used by the known implementations.
The rest of the primitives are being ignored.
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">
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<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>
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<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">
<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"
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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>
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.
7. Examples
7.1. The Fast Update command for the full picture
In the following example the full picture "Fast Update" command is
issued towards the remote video decoder(s). Note that this primitive
is supported by all known implementaions.
<?xml version="1.0" encoding="utf-8" ?>
<media_control>
<vc_primitive>
<to_encoder>
<picture_fast_update/>
</to_encoder>
</vc_primitive>
</media_control>
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7.2. Reporting an error
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>
7.3. Freeze and Fast Update commands for the full picture
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>
7.4. Advanced XML schema usage
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" [7] values "1" and
"2". The GOB "Fast Update" command is issued towards the remote
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video encoder to be applied to a video stream with "mid" value "3".
<?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>
8. Transport
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. This document
registers the defined schema with IANA according to the guidelines
specified in [9] .
9. IANA Considerations
Application/media_control+xml MIME type
MIME Media Type Name: application
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MIME subtype name: media_control+xml
Required parameters: None
Optional parameters: charset
See RFC 3023 [5] 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 MIME type are discussed in RFC xxxx [[Note to RFC editor:
replace xxxx with the RFC number of this document when
published]]. RFC 1874 [1] and RFC 3023 [5] 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
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.
10. URN for XML schema
URI: urn:ietf:params:xml:ns:media_control
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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
11. Security Considerations
This document doesn't introduce new security considerations beyond
covered in [4].
12. Changes History
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12.1. Changes since -03
o This version reflects the deployment experience since the defined
mechanism has been implemented and tested among the vendors
represented by the authors of this document.
o The XML schema is identical to version -03.
12.2. Changes since -02
o This version contains editorial changes only.
o The XML schema is identical to version -02.
13. References
13.1. Normative References
[1] Levinson, E., "SGML Media Types", RFC 1874, December 1995.
[2] Turletti, T., "RTP Payload Format for H.261 Video Streams",
RFC 2032, October 1996.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.
[5] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
RFC 3023, January 2001.
[6] 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.
[7] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
[8] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[9] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
13.2. Informative References
[10] Ott, J. and S. Wenger, "Extended RTP Profile for RTCP-based
Feedback(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-11 (work in
progress), August 2004.
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[11] Wenger, S., "Codec Control Messages in the Audio-Visual Profile
with Feedback (AVPF)", draft-wenger-avt-avpf-ccm-02 (work in
progress), February 2006.
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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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