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Network Working Group                                           O. LevinRequest for Comments: 5168                         Microsoft CorporationCategory: Informational                                          R. Even                                                                 Polycom                                                            P. Hagendorf                                                               RADVISION                                                              March 2008XML Schema for Media ControlStatus 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 2008Table of Contents1. Introduction ....................................................22. Conventions .....................................................23. Background ......................................................34. The Video Control Commands ......................................35. The Schema Definition ...........................................46. Error Handling ..................................................57. Examples ........................................................57.1. The Fast Update Command for the Full Picture ...............57.2. Reporting an Error .........................................58. Transport .......................................................69. IANA Considerations .............................................69.1. Application/media_control+xml Media Type Registration ......610. Security Considerations ........................................711. References .....................................................811.1. Normative References ......................................811.2. Informative References ....................................81.  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 inRFC 2119 [2].Levin, et al.                Informational                      [Page 2]

RFC 5168                     Media Control                    March 20083.  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 Update5.  The Schema Definition   <?xml version="1.0" encoding="utf-8" ?>   <xs:schema    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 toRFC 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.  Examples7.1.  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.  SeeRFC 3023 [4], Section 3.2.   Security considerations:   Security considerations specific to uses      of this type are discussed inRFC 5168.RFC 1874 [1] andRFC 3023      [4] discuss security issues common to all uses of XML.   Interoperability considerations:   None.   Published specification:RFC 5168Levin, 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 inSection 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 200811.  References11.1.  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",RFC3023, 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 2008Authors' 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.comLevin, et al.                Informational                      [Page 9]

RFC 5168                     Media Control                    March 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Levin, et al.                Informational                     [Page 10]