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rfc:rfc5168

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.

Levin, et al. Informational [Page 1] RFC 5168 Media Control March 2008

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].

Levin, et al. Informational [Page 2] RFC 5168 Media Control March 2008

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.

Levin, et al. Informational [Page 3] RFC 5168 Media Control March 2008

 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>

Levin, et al. Informational [Page 4] RFC 5168 Media Control March 2008

 </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.

Levin, et al. Informational [Page 5] RFC 5168 Media Control March 2008

 <?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

Levin, et al. Informational [Page 6] RFC 5168 Media Control March 2008

 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.

Levin, et al. Informational [Page 7] RFC 5168 Media Control March 2008

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.

Levin, et al. Informational [Page 8] RFC 5168 Media Control March 2008

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

Levin, et al. Informational [Page 9] RFC 5168 Media Control March 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Levin, et al. Informational [Page 10]

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