Internet Engineering Task Force Audio-Video Transport Working Group
INTERNET-DRAFT H. Schulzrinne
AT&T Bell Laboratories
July 19, 1993
Expires: 10/01/93
Sample Profile for the Use of RTP for Audio and Video Conferences with
Minimal Control
Status of this Memo
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Abstract
This note describes a profile for the use of the real-time
transport protocol (RTP) and the associated control protocol, RTCP,
within audio and video multiparticipant conferences with minimal
control. It provides interpretations of generic fields within the
RTP specification suitable for audio and video conferences. In
particular, this document defines a set of default mappings from
content index to encodings.
1 Introduction
This profile defines aspects of RTP left unspecified in the RTP protocol
definition (RFC TBD). This profile is intended for the use within audio and
INTERNET-DRAFT AV Profile July 19, 1993
video conferences with minimal session control. In particular, no support
for the negotiation of parameters or admission control is provided. Other
profiles may make different choices for the items specified here. The
profile specifies the use of RTP over unicast and multicast UDP as well
as ST-II. For unicast UDP and ST-II, references to multicast addresses
are to be ignored. The use of this profile is indicated by the use of
a media-specific well-known port number. The profile may also be used
with other port numbers. For example, the use of a particular session
announcement tool could imply use of this profile.
2 Multiplexing and Demultiplexing
Packets sharing the same multicast group address, the same destination port
number and the same flow value belong to the same conference. Within a
conference, a packet is mapped to a site (state) through its synchronization
source identifier and network source port. Unless otherwise defined, flow 0
is used for audio and flow 1 for video.
3 FMT
The content field within the FMT option describes the media encoding used.
The four octets contain one of the encodings defined by the Internet
Assigned Numbers Authority (IANA) or an encoding agreed upon by mutual
consent of all conference participants. The names and the defined codings
are defined in RFC TBD [media] and encoded in US-ASCII. Case is significant.
If the name is shorter than four characters, it is padded with one or more
space characters (ASCII 32 decimal). Experimental encodings should start
with the letter 'X'.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| FMT | length |0|0| content | clock quality |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name of encoding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| channels | sampling rate (Hz) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... encoding specific parameters ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: FMT for Audio
For audio encodings, the index into the table of encodings is followed by
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| FMT | length |0|0| content | clock quality |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name of video encoding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | encoding-specific parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... encoding-specific parameters ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: FMT for Video
a field containing a channel count and a sample rate field, measured in
samples per second.(1) A channel count of zero is considered invalid. A
packetization interval of 20 ms or a multiple thereof is suggested as it
leads to integral sample counts for all common sampling rates.
For video encodings, a one-octet numeric version identifier further
describes the encoding. Unless otherwise defined, the version identifier
has the value zero.
4 Standard Encodings
Unless specified with the FMT option, the mapping between the content
field in an RTP packet and encodings, sampling rates and channel counts is
specified by Tables 1 and 2. Values of 31 and below cannot be redefined by
FMT options. In other words, only values of 32 and above are valid in the
content field within an FMT option. The receiver is expected to discard RTP
packets containing media data with unknown content field values. Sites are
expected to keep the mapping between content and encoding constant, so that
lost packets containing FMT options do not lead the receiver to misinterpret
media data. Additional standard encodings are defined by the Internet
Assigned Numbers Authority.
------------------------------
1. Fractional samples per second was considered excessive as the typical
crystal accuraccy of 100 ppm translates into about one Hz or more of
sampling rate inaccuracy.
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index encoding sampling rate channels
________name______________(kHz)___________
0 PCMU 8 1
1 1016 8 1
2 G721 8 1
3 GSM 8 1
4 G723 8 1
5 DVI 8 1
6 L16 16 1
_____7__L16__________________48________2__
Table 1: Standard Audio Encodings
_number__name_
31 H261
30 Bolt
29 dvc
28 nv
27 CUSM
26 JPEG
Table 2: Standard Video Encodings
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5 Port Assignments and Miscellaneous
ST-II SAPs and UDP ports 4443 and 4444 are to be used as the destination
for multicast real-time audio and video data carried by RTP, respectively.
Unicast connections may use the this or a set of mutually agreed-upon port
numbers.
RTCP messages should be sent periodically, with a randomly varying period to
avoid synchronization. A period of between 3 and 10 seconds is suggested,
increased to limit the total network bandwidth and host interrupt load to
some small percentage of that attributable to media data.
6 Address of Author
Henning Schulzrinne
AT&T Bell Laboratories
MH 2A244
600 Mountain Avenue
Murray Hill, NJ 07974
telephone: +1 908 582 2262
electronic mail: hgs@research.att.com
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