Network Working Group O. Levin
Request for Comments: 5168 Microsoft Corporation
Category: Informational R. Even
Polycom
P. Hagendorf
RADVISION
March 2008
XML Schema for Media Control
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document defines an Extensible Markup Language (XML) Schema for
video fast update in a tightly controlled environment, developed by
Microsoft, Polycom, Radvision and used by multiple vendors. This
document describes a method that has been deployed in Session
Initiation Protocol (SIP) based systems over the last three years and
is being used across real-time interactive applications from
different vendors in an interoperable manner. New implementations
are discouraged from using the method described except for backward
compatibility purposes. New implementations are required to use the
new Full Intra Request command in the RTP Control Protocol (RTCP)
channel.
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Table of Contents
1. Introduction ....................................................2
2. Conventions .....................................................2
3. Background ......................................................3
4. The Video Control Commands ......................................3
5. The Schema Definition ...........................................4
6. Error Handling ..................................................5
7. Examples ........................................................5
7.1. The Fast Update Command for the Full Picture ...............5
7.2. Reporting an Error .........................................5
8. Transport .......................................................6
9. IANA Considerations .............................................6
9.1. Application/media_control+xml Media Type Registration ......6
10. Security Considerations ........................................7
11. References .....................................................8
11.1. Normative References ......................................8
11.2. Informative References ....................................8
1. Introduction
This document defines an Extensible Markup Language (XML) Schema for
video fast update request in a tightly controlled environment,
developed by Microsoft, Polycom, Radvision and used by multiple
vendors. Implementation of this schema for interactive video
applications in Session Initiation Protocol (SIP) [5] environments
was designed in order to improve user experience. This mechanism is
being used by both end user video conferencing terminals and
conferencing servers in shipping products. This document describes
the current method, but new implementations are discouraged from
using this method, except for backward compatibility with legacy
systems. Shipping products and new products SHOULD use the Full
Intra Request, described in [7].
Sending video fast update using the SIP signaling path, as described
in this document, is inferior to using the RTP Control Protocol
(RTCP) feedback method [7], since the command flows through all the
proxies in the signaling path adding delay to the messages and
causing unnecessary overload to the proxies. RTCP messages flow
end-to-end and not through the signaling proxies. The RTCP feedback
document [7] also adds other required control functions, such as the
flow control command, which is missing from this document.
2. 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 [2].
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3. Background
SIP typically uses the Real-time Transport Protocol (RTP) [6] for the
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 [8] has been introduced in order to
improve basic RTCP feedback time in case of loss conditions across
different coding schemes. This technique addresses signaling of loss
conditions and the recommended recovery steps.
Just recently, an extension to the feedback mechanism has been
proposed [7] 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 a technology that has been deployed in
SIP-based systems over the last three years and is being used across
real-time interactive applications from different vendors in an
interoperable manner. This memo documents this technology for the
purpose of describing current practice and new implementation MUST
use the RTCP Full Intra Request command specified in the RTCP-based
codec control messages document[7].
4. The Video Control Commands
Output of a video codec is a frame. The frame can carry complete
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.
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 information about a "full"
picture.
An "Intra" frame may be of large size. In order to prevent causing
network congestion, the current media capacity and network conditions
MUST be validated before sending an "Intra" frame when receiving a
fast update command, defined in this document.
In order to meet the presented requirements, a video primitive is
defined by this document.
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The following command is sent to the remote encoder:
o Video Picture Fast Update
5. The Schema Definition
<?xml version="1.0" encoding="utf-8" ?>
<xs:schema id="TightMediaControl"
elementFormDefault="qualified"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="media_control">
<xs:complexType>
<xs:sequence>
<xs:element name="vc_primitive"
type="vc_primitive"
minOccurs="0"
maxOccurs="unbounded" />
<xs:element name="general_error"
type="xs:string"
minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
</xs:element>
<!-- Video control primitive. -->
<xs:complexType name="vc_primitive">
<xs:sequence>
<xs:element name="to_encoder" type="to_encoder" />
<xs:element name="stream_id"
type="xs:string"
minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
<!-- Encoder Command:
Picture Fast Update
-->
<xs:complexType name="to_encoder">
<xs:choice>
<xs:element name="picture_fast_update"/>
</xs:choice>
</xs:complexType>
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</xs: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 the indication of
errors other than those 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.
When receiving the general error response, the user agent client
(UAC) that sent the request SHOULD NOT send further fast update
requests in the current dialog.
According to RFC 2976 [3], the only allowable final response to a SIP
INFO containing a message body is a 200 OK. If SIP INFO is used to
carry the request, the error message should be carried in a separate
INFO request.
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).
<?xml version="1.0" encoding="utf-8" ?>
<media_control>
<vc_primitive>
<to_encoder>
<picture_fast_update/>
</to_encoder>
</vc_primitive>
</media_control>
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.
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<?xml version="1.0" encoding="utf-8" ?>
<media_control>
<general_error>
Parsing error: The original XML segment is:...
</general_error>
</media_control>
8. Transport
The defined XML document is conveyed using the SIP INFO method [3]
with the "Content-Type" set to "application/media_control+xml". This
approach benefits from the SIP built-in reliability.
9. IANA Considerations
This document registers a new media type.
9.1. Application/media_control+xml Media Type Registration
Type name: application
Subtype name: media_control+xml
Required parameters: None
Optional parameters: charset
Indicates the character encoding of enclosed XML.
Encoding considerations: 8bit
Uses XML, which can employ 8-bit characters, depending on the
character encoding used. See RFC 3023 [4], Section 3.2.
Security considerations: Security considerations specific to uses
of this type are discussed in RFC 5168. RFC 1874 [1] and RFC 3023
[4] discuss security issues common to all uses of XML.
Interoperability considerations: None.
Published specification: RFC 5168
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Applications that 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.
Person and email address to contact for further information:
Name: Roni Even
E-Mail: even.roni@gmail.com
Intended usage: LIMITED USE
Restrictions on usage: None.
Author: Roni Even. <even.roni@gmail.com>
Change Controller: Roni Even. <even.roni@gmail.com>
10. Security Considerations
The video control command transported using the method described in
the document may cause the sender of the video data to send more data
within the allowed bandwidth, as described in Section 4.
This document defines one control message; changing the content of
the message will cause the video sender to ignore the request and
send an error response. This may prevent the display of a video
stream. The control message itself does not carry any sensitive
information.
An eavesdropper may inject messages or change them, which may lead to
either more data on the network or loss of video image. Using data
integrity validation will prevent adding or changing of messages.
If the video media is sent over a secure transport, it is recommended
to secure the signaling using TLS as explained in [5]. In most
cases, securing the media will require a secure signaling path.
The security considerations of [3] and [5] apply here.
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11. References
11.1. Normative References
[1] Levinson, E., "SGML Media Types", RFC 1874, December 1995.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.
[4] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC
3023, January 2001.
[5] 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.
[6] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[7] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec
Control Messages in the RTP Audio-Visual Profile with Feedback
(AVPF)", RFC 5104, February 2008.
11.2. Informative References
[8] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control Protocol
(RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 2006.
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Authors' Addresses
Orit Levin
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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