RFC4573: MIME Type Registration for RTP Payload Format for H.224

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Network Working Group                                            R. Even
Request for Comments: 4573                                   A. Lochbaum
Category: Standard Track                                         Polycom
                                                               July 2006

        MIME Type Registration for RTP Payload Format for H.224

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   In conversational video applications, far-end camera control protocol
   is used by participants to control the remote camera.  The protocol
   that is commonly used is ITU H.281 over H.224.  The document
   registers the H224 media type.  It defines the syntax and the
   semantics of the Session Description Protocol (SDP) parameters needed
   to support far-end camera control protocol using H.224.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Far-End Camera Control Protocol .................................2
   4. IANA Considerations .............................................2
      4.1. Media Type Registration ....................................2
   5. SDP Parameters ..................................................4
      5.1. Usage with the SDP Offer Answer Model ......................4
   6. Security Considerations .........................................5
   7. References ......................................................5
      7.1. Normative References .......................................5
      7.2. Informative References .....................................6

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1.  Introduction

   The document registers the H224 media type, which may be used by
   systems that use SDP [RFC4566].

   This media type is used for supporting the simple far-end camera
   control protocol on SDP-based systems.  The media type helps
   signaling gateways between H.323 [ITU.H323] and SDP-based systems to
   use far-end camera control, end to end, without any protocol
   translation in the middle.

   The document defines the H224 media type since the RTP packets in
   H.323 annex Q [ITU.H323] carry H.224 frames [ITU.H224].  The far-end
   camera control protocol (FECC) is internal to the H.224 frame and is
   identified by the client ID field of the H.224 packet.

   The document will define the SDP [RFC4566] parameters needed to
   support the above far-end camera control protocol in systems that use

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [RFC2119] and
   indicate requirement levels for compliant RTP implementations.

3.  Far-End Camera Control Protocol

   This simple protocol is based on ITU-T H.281[ITU.281] frames carried
   in ITU-T H.224 packets in an RTP/UDP channel.  H.323 annex Q
   specifies how to build the RTP packets from the H.224 packets.

   Using far end camera control protocol in point-to-point calls and
   multipoint calls for packet-switch networks is described in H.323,
   annex Q.

4.  IANA Considerations

4.1.  Media Type Registration

   This section describes the media types and names associated with this
   payload format.  The registration uses the templates defined in RFC
   4288 [RFC4288].  It follows RFC 3555 [RFC3555].

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4.1.1.  Registration of MIME Media Type application/h224

   MIME media type name: application

   MIME subtype name: H224

   Required parameters: None

   Optional parameters: None

   Encoding considerations:

      This media type is framed (see H.323, Annex Q [ITU.H323]) and
      contains binary data; see Section 4.8 of [RFC4288]

   Security considerations: See Section 6 of RFC 4573.

   Interoperability considerations:

      Terminals sending simple far-end camera control commands should
      use this MIME type.  Receivers who cannot support the protocol
      will reject the channel.

   Published specification: RFC 4573

   Applications that use this media type:

      Video conferencing applications.

   Additional information: None

   Person and email address to contact for further information:

      Roni Even: roni.even@polycom.co.il

   Intended usage: COMMON

   Restrictions on usage:

      This media type depends on RTP framing and thus is only defined
      for transfer via RTP [RFC3550].  Transport within other framing
      protocols is not defined at this time.

   Author: Roni Even

   Change controller:

      IETF Audio/Video Transport working group, delegated from the IESG.

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5.  SDP Parameters

   The media type application/h224 string is mapped to fields in the
   Session Description Protocol (SDP) as follows:

      o The media name in the "m=" line of SDP MUST be application.  The
      transport SHALL be any applicable RTP profile (for example RFC
      3551 [RFC3551]), and the payload type is dynamic.

      o The encoding name in the "a=rtpmap" line of SDP MUST be h224
      (the MIME subtype).

      o The default clock rate in the "a=rtpmap" line MUST be 4800.

   The recommended maximum bandwidth for this protocol is 6.4 kbit/sec.

5.1.  Usage with the SDP Offer Answer Model

   When offering FECC using SDP in an Offer/Answer model [RFC3264], the
   following considerations are necessary.

   Far-end camera control communication is uni-directional.  H.224 is
   bi-directional and can be used to learn the capabilities of the
   remote video end point, e.g., how many cameras it has.  The offer
   answer exchange is dependent on the functionality of both sides.

   The offerer offers a sendonly channel if its camera cannot be
   remotely controlled and if the offerer does not intend to use H.224
   to learn the capabilities of the remote video endpoints.

   In all other cases, when the offerer's camera can be remotely
   controlled and/or it intends to use H.224 capabilities negotiation,
   the offerer offers a sendrecv channel.

   The answerer behavior is as follows:

   If it receives an offer with sendonly, it answers with a recvonly if
   it supports far-end camera control; otherwise, it ignores/rejects the

   If it receives an offer with sendrecv and its camera can be remotely
   controlled, or it intends to use H.224 capabilities negotiation, it
   answers with a sendrecv option.  If its camera cannot be remotely
   controlled, it can answer with a sendonly attribute.  The answerer
   may also reject the offer if he does not support FECC or does not
   intend to use FECC at the moment.

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6.  Security Considerations

   H.224 payload format, defined in H.323, annex Q defines packet
   structure based on RTP using the RTP header structure from RFC 3550.
   Those packets are subject to the security considerations discussed in
   the RTP specification [RFC3550].  This implies that confidentiality
   of the media streams is achieved by encryption.  Secure Realtime
   Transport Protocol (SRTP) [RFC3711] may be used to provide both
   encryption and integrity protection of RTP flow.

   A potential denial-of-service threat exists for data that causes
   application behavior like camera movement.  The attacker can inject
   pathological datagrams into the stream that cause the receiver to
   change the camera position.  Therefore, the usage of data origin
   authentication and data integrity protection of at least the H.323
   annex Q packet is RECOMMENDED; for example, with SRTP.

   Note that the appropriate mechanism to ensure confidentiality and
   integrity of H.323 annex Q packets and their payloads is very
   dependent on the application and on the transport and signaling
   protocols employed.  Thus, although SRTP is given as an example
   above, other possible choices exist.

7.  References

7.1.  Normative References

   [ITU.281]  International Telecommunications Union, "A far end camera
              control protocol for videoconferences using H.224", ITU- T
              Recommendation H.281, November 1994.

   [ITU.H224] International Telecommunications Union, "A real time
              control protocol for simplex applications using the H.221
              LSD/HSD/HLP channels.", ITU-T Recommendation H.224,
              February 2000.

   [ITU.H323] International Telecommunications Union, "Visual telephone
              systems and equipment for local area networks which
              provide a non-guaranteed quality of service", ITU-T
              Recommendation H.323, July 2003.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June

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   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

7.2.  Informative References

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC3555]  Casner, S. and P. Hoschka, "MIME Type Registration of RTP
              Payload Formats", RFC 3555, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

Authors' Addresses

   Roni Even
   94 Derech Em Hamoshavot
   Petach Tikva  49130

   EMail: roni.even@polycom.co.il

   Andrew Lochbaum
   6500 River Place Blvd, Building 6
   Austin, TX  78730

   EMail: alochbaum@polycom.com

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RFC 4573                          FECC                         July 2006

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