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).
Abstract
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
SDP.
2. Terminology
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 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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RFC 4573 FECC July 2006
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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RFC 4573 FECC July 2006
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
offer.
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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RFC 4573 FECC July 2006
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
2002.
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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
Polycom
94 Derech Em Hamoshavot
Petach Tikva 49130
Israel
EMail: roni.even@polycom.co.il
Andrew Lochbaum
Polycom
6500 River Place Blvd, Building 6
Austin, TX 78730
USA
EMail: alochbaum@polycom.com
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RFC 4573 FECC July 2006
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