AVT R. Even
Internet-Draft Polycom
Expires: April 7, 2004 P. Koskelainen
Nokia
October 8, 2003
SDP parameters for supporting H.263 and H.261 options
draft-even-avt-h263-h261-options-00.txt
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines the syntax and the semantics of SDP parameters
needed to support features of H.261 and H.263 video codecs. This
optional parameters enables the H.261 and H.263 codecs to specify
their functionality enabling better video support. The document
specify codec specific parameters that can be used with SIP, SAP or
any similar protocol that uses SDP to signal support of those codecs.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. H.261 Codec Parameters . . . . . . . . . . . . . . . . . . . . . 3
3. H.263 Codec Parameters . . . . . . . . . . . . . . . . . . . . . 4
4. Usage of SDP H.261 and H.263 options with SIP . . . . . . . . . 6
5. Use of SDP H.263 and H.261 options with SAP . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . 9
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1. Introduction
Internet multimedia conferencing is picking up with point-to-point
and multipoint video conferencing implementations. The common video
codecs that are used in video conferencing applications include
H.261[3] and the basic H.263 with its two revisions from 1998 and
2000[4]. The current usage of H.261[3] and H.263[4] is limited since
the current SDP MIME types defined in RFC3555 [6] are defining only
part of the functionality needed.
This document specifies the SDP syntax and the semantics for
describing the optional H.261 and H.263 features. The syntax in this
document uses the SDP "a=fmtp" attribute which is meant to carry
codec specific parameters. These codec specific parameters can be
used with SIP[1], SAP[2] or any similar protocol that uses SDP to
signal support of those codecs.
2. H.261 Codec Parameters
For further description of these H.261 parameters refer to the ITU
H.261 document[3].
The fmtp parameters for H.261 are represented in the SDP attribute.
a=fmtp:xx H261_option
The xx is the RTP payload number, it can be a static or dynamic
payload number. The recommendation is to use dynamic payload number.
The syntax of H261_option is
H261_option = *Size SP | *Annex SP
Size = "QCIF" "=" "mpi | "CIF" "=" mpi
mpi = 1*2DIGIT
Annex = "D"
The Size parameter specifies the picture size supported and at what
frame rate. H.261 defines two resolution QCIF and CIF.
mpi is an integer value (1..4) and it means the maximum picture frame
rate is (29.97/mpi) frames/sec. The definition of the mpi is
according to the video codec specification.
Annex D specifies support for still image graphics according to H.261
annex D.
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3. H.263 Codec Parameters
H.263 has three MIME types defined in RFC3555. The parameters in
this section may be used with any of the H263 MIME types. When used
with H.263-2000 MIME subtype name, the optional parameters profile
and levels must not be used. For description of these H.263
parameters refer to the ITU H.263 document[4].
The fmtp parameters for H.263 are represented in the SDP attribute.
a=fmtp:xx H263_option
The xx is the RTP payload number, it can be a static or dynamic
payload number. The recommendation is to use dynamic payload number.
The syntax of H263_option is
H263_option = *Size SP | *Annex SP | *Params
Size = "SQCIF" "=" "mpi | "QCIF" "=" mpi | "CIF" "=" mpi |
"CIF4" "=" mpi I "CIF16" "=" mpi |
"XMAX" "=" xmax SP "YMAX" "=" ymax SP "MPI" "=" mpi
mpi = 1*2DIGIT
xmax=1*4DIGIT
ymax=1*4DIGIT
The Size parameter specifies the picture size supported and at what
frame rate. H.263 defines fixed resolution SQCIF, QCIF, CIF, 4xCIF
and 16xCIF as well as custom picture size that are presented by their
maximum X and Y size. The X and Y values must be dividable by 4.
mpi is an integer value (1..32) and it means the maximum picture
frame rate is (29.97/mpi) frames/sec. The definition of the mpi is
according to the video codec specification.
Annex = "D" "=" #annex_d | "E" | "F" | "G" | "I" | "J" |
"K" "=" #annex_k | "L" "=" #annex_l | "M" | "N" "=" annex_n |
"O" "=" #annex_o | "P" | "Q" | "R" | "S" | "T"
annex_d= "1" | "2"
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annex_k= "1" | "2" | "3" | "4"
annex_l= "1" | "2" | "3" | "4" | "5" | "6" | "7"
annex_n= "1" | "2" | "3" | "4"
annex_o= "1" | "2" | "3"
Here #term means comma separated list of term.
The H.263 parameters that can be specified are in the Params options.
Params= "PAR" "=" par_a ":" par_b | "CPCF" "=" cpcf | "MaxBR" "="
maxbr |
"BPP" "=" bpp | "HRD" | "Interlaced"
par_a=1*3DIGIT
par_b=1*DIGIT
cpcf=1*2DIGIT "." 1*3DIGIT
maxbr=1*5DIGIT
bpp=1*5DIGIT
Explanations:
Arbitrary Pixel Aspect Ratio (PAR):
Par_a and par_b are integers between 0 and 255.
Default ratio is 12:11 if not otherwise specified.
Arbitrary (Custom) Picture Clock Frequency (CPCF):
Cpcf is floating point value. Default value is 29.97.
MaxBitRate (MaxBR):
Maximum video stream bitrate, presented with units of 100 bits/s.
MaxBR value is an integer between 1..19200.
BitsPerPictureMaxKb (BPP):
Maximum amount of kilobits allowed to represent a single picture
frame, value is specified by largest supported picture resolution,
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see [1]. If this parameter is not present, then default value, that
is based on the maximum supported resolution, is used. BPP is
integer value between 0 and 65536.
Hypothetical Reference Decoder (HRD):
See annex B of H.263 specification.
Interlaced or 60 fields indicates the support for interlace display
according to H.263 annex W.6.3.11
These parameters are separated by space.
4. Usage of SDP H.261 and H.263 options with SIP
This document does not specify actual SIP signaling. The decoder
send its preferred parameters and let the other end select according
to SIP procedures. This syntax may be sent, for example, with SIP
INVITE and corresponding status response (200 ok). Other SIP methods
may be used.
Codec options: (D,E,F,G,I,J,K,L,M,N,O,P,Q,R,S,T) These characters
exist only if the sender of this SDP message is able or willing to
decode those. E.g. If a terminal is capable of decoding Syntax
based Arithmetic Coding (SAC) and Advanced Prediction (AP) options,
it can put E F in the end of the format specific parameters.
Picture sizes and MPI:
Supported picture sizes and their corresponding minimum picture
interval (MPI) information for H.261 and H.263 can be combined. All
picture sizes can be advertised to other party, or only some subset
of it. Terminal announces only those picture sizes (with their MPIs)
which it is willing to receive. For example, MPI=2 means that
maximum (decodeable) picture rate per second is about 15.
Parameters occurring first are the most preferred picture mode to be
received.
Example of the usage of these parameters:
CIF=4 QCIF=3 SQCIF=2 XMAX=360 YMAX=240 MPI=2
This means that sender hopes to receive CIF picture size, which it
can decode at MPI=4. If that is not possible, then QCIF with MPI
value 3, if that is neither possible, then SQCIF with MPI value =2.
It is also allowed (but least preferred) to send arbitrary picture
sizes (max 360x240) with MPI=2. Note that most encoders support at
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least QCIF and CIF fixed resolutions and they are expected to be
available almost in every H.261 and H.263 based video application.
MaxBR and BPP parameters:
Both these parameters are useful in SIP. MaxBitRate is video decoder
property, hence it differs from SDP b : bandwidth-value attribute
which refers more to application's total bandwidth (an application
consists often of both audio and video).
BitsPerPictureMaxKb is needed especially for decoder buffer size
estimation to reduce the probability of video buffer overflow.
Below is an example of H.263 SDP syntax in SIP message.
a=fmtp: xx CIF=4 QCIF=2 MaxBR=1000 E F
This means that the sender of this message can decode H.263 bit
stream with following options and parameters: Preferred resolution is
CIF (its MPI is 4), but if that is not possible then QCIF size is ok.
Maximum receivable bitrate is 100 kbit/s (1000*100 bit/s) and SAC and
AP options can be used.
5. Use of SDP H.263 and H.261 options with SAP
SAP announcements are one-way only. All H.263/H.261 options can be
used to signal that the sending terminal is going to use these
options in its transmitted video stream. It is just an informal
message.
Usually only one picture size (with its MPI) exists. However, since
it is possible for a video source (terminal) to change its picture
size during session, several picture sizes can exist in the parameter
list. First one is the original picture size to be used in the
beginning of the session.
6. Security Considerations
The security for these features are the same as for SDP.
References
[1] 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.
[2] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
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[3] International Telecommunications Union, "Video codec for
audiovisual services at p x 64 kbit/s", ITU-T Recommendation
H.261, March 1993.
[4] International Telecommunications Union, "Video coding for low
bit rate communication", ITU-T Recommendation H.263, February
1998.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[6] Casner, S. and P. Hoschka, "MIME Type Registration of RTP
Payload Formats", RFC 3555, July 2003.
Authors' Addresses
Roni Even
Polycom
94 Derech Em Hamoshavot
Petach Tikva 49130
Israel
EMail: roni.even@polycom.co.il
Petri Koskelainen
Nokia
P.O. Box 100 (Visiokatu 1)
Tampere FIN-33721
Finland
EMail: petri.koskelainen@nokia.com
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