semicolon-separated list of parameters
6.2.1. Usage with the SDP Offer Answer Model
When H.261 is offered over RTP using SDP in an Offer/Answer model
[RFC3264] the following considerations are necessary.
Codec options: (D) This option MUST NOT appear unless the sender of
this SDP message is able to decode this option. This option SHALL be
considered a receiver’s capability even when it is sent in a
"sendonly" offer.
Picture sizes and MPI:
Supported picture sizes and their corresponding minimum picture
interval (MPI) information for H.261 can be combined. All picture
sizes may be advertised to the other party, or only a subset of it.
Using the recvonly or sendrev direction attribute, a terminal SHOULD
announce those picture sizes (with their MPIs) that it is willing to
receive. For example, CIF=2 means that receiver can receive a CIF
picture and that the frame rate SHALL be less then 15 frames per
second.
When the direction attribute is sendonly, the parameters describe the
capabilities of the stream that the sender can produce.
Implementations following this specification SHALL specify at least
one supported picture size.
If the receiver does not specify the picture size/MPI parameter, then
it is safe to assume that it is an implementation that follows RFC
2032. In that case, it is RECOMMENDED to assume that such a receiver
is able to support reception of QCIF resolution with MPI=1.
Parameters offered first are the most preferred picture mode to be
received.
An example of media representation in SDP is as follows CIF at 15
frames per second, QCIF at 30 frames per second and annex D
m=video 49170/2 RTP/AVP 31
a=rtpmap:31 H261/90000
a=fmtp:31 CIF=2;QCIF=1;D=1
This means that the sender of this message can decode an H.261 bit
stream with the following options and parameters: preferred
resolution is CIF (its MPI is 2), but if that is not possible, then
QCIF size is also supported. Still image using annex D MAY be used.
7. Backward Compatibility to RFC 2032
The current document replaces RFC 2032. This section will address
the major backward compatibility issues.
7.1. Optional H.261-Specific Control Packets
RFC 2032 defined two H.261-specific RTCP control packets, "Full
INTRA-frame Request" and "Negative Acknowledgement". Support of
these control packets was optional. The H.261-specific control
packets differ from normal RTCP packets in that they are not
transmitted to the normal RTCP destination transport address for the
RTP session (which is often a multicast address). Instead, these
control packets are sent directly via unicast from the decoder to the
encoder. The destination port for these control packets is the same
port that the encoder uses as a source port for transmitting RTP
(data) packets. Therefore, these packets may be considered "reverse"
control packets. This memo suggests generic methods to address the
same requirement. The authors of the documents are not aware of
products that support these control packets. Since these are
optional features, new implementations SHALL ignore them, and they
SHALL NOT be used by new implementations.
7.2. New SDP Optional Parameters
The document adds new optional parameters to the H261 payload type.
Since these are optional parameters, we expect that old
implementations ignore these parameters, whereas new implementations
that receive the H261 payload type capabilities with no parameters
will assume that it is an old implementation and will send H.261 at
QCIF resolution and 30 frames per second.
8. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550], and in any appropriate RTP profile (e.g.,
[RFC3551]). This implies that confidentiality of the media streams
is achieved by encryption. SRTP [RFC3711] may be used to provide
both encryption and integrity protection of RTP flow. Because the
data compression used with this payload format is applied end to end,
encryption will be performed after compression, so there is no
conflict between the two operations.
A potential denial-of-service threat exists for data encoding using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the stream that are complex to decode and cause the receiver to
be overloaded. The usage of authentication of at least the RTP
packet is RECOMMENDED. H.261 is vulnerable to such attacks because
it is possible for an attacker to generate RTP packets containing
frames that affect the decoding process of future frames. Therefore,
the usage of data origin authentication and data integrity protection
of at least the RTP packet is RECOMMENDED; for example, with SRTP.
Note that the appropriate mechanism to ensure confidentiality and
integrity of RTP 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.
9. Acknowledgements
This is to acknowledge the authors of RFC 2032, Thierry Turletti and
Christian Huitema. Special thanks for the work done by Petri
Koskelainen from Nokia and Nermeen Ismail from Cisco, who helped with
drafting the text for the new MIME types.
10. Changes from RFC 2032
The changes from the RFC 2032 are:
1. The H.261 MIME type is now in the payload specification.
2. Added optional parameters to the H.261 MIME type
3. Deprecated the H.261 specific control packets
4. Editorial changes to be in line with RFC editing procedures
11. References
11.1. Normative References
[H261] International Telecommunications Union, "Video codec for
audiovisual services at px 64 kbit/s", ITU Recommendation
H.261, March 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
11.2. Informative References
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288,
December 2005.
[RFC4585] 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.
[ITU.H245] International Telecommunications Union, "CONTROL PROTOCOL
FOR MULTIMEDIA COMMUNICATION", ITU Recommendation H.245,
2003.
[INRIA] Turletti, T., "H.261 software codec for videoconferencing
over the Internet", INRIA Research Report 1834,
January 1993.
[IVS] Turletti, T., "INRIA Videoconferencing tool (IVS)",
available by anonymous ftp from zenon.inria.fr in the
"rodeo/ivs/last_version" directory. See also URL
<http://www.inria.fr/rodeo/ivs.html>.
[BT601] International Telecommunications Union, "Studio encoding
parameters of digital television for standard 4:3 and
wide-screen 16:9 aspect ratios", ITU-R Recommendation
BT.601-5, October 1995.
[MICE] Sasse, MA., Bilting, U., Schultz, CD., and T. Turletti,
"Remote Seminars through MultiMedia Conferencing:
Experiences from the MICE project", Proc. INET’94/JENC5,
Prague pp. 251/1-251/8, June 1994.
[VIC] MacCanne, S., "VIC Videoconferencing tool, available by
anonymous ftp from ee.lbl.gov in the "conferencing/vic"
directory".
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[H221] International Telecommunications Union, "Frame structure
for a 64 to 1920 kbit/s channel in audiovisual
teleservices", ITU Recommendation H.221, May 1999.
Author’s Address
Roni Even
Polycom
94 Derech Em Hamoshavot
Petach Tikva 49130
Israel
EMail: roni.even@polycom.co.il
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