Subject: [AVT] question: mapping dynamic RTP formats using SDP

From: "Hager, Todd" <[email-masked]>
Date: Fri, 28 Feb 2003 17:17:54 -0800

Subject: question: mapping dynamic RTP formats using SDP
From: "Hager, Todd" <[email-masked]>
To: <[email-masked]>
Date: Fri, 28 Feb 2003 17:17:54 -0800

Hi all,
 
What is the best method to define a valid encoding name for a new
encoding format?
 
Regarding <draft-flaks-avt-rtp-ac3-03.txt>, I'm trying to determine the
best method of choosing a value for the payload type for AC-3 in the RTP
header.
 
I am proceeding under the assumption that the best course is to randomly
choose a value from the dynamic range (96-127) described in <RTP Profile
for Audio and Video Conferences with Minimal Control.txt> (RFC 1890),
and use SDP to somehow map this value to the AC-3 encoding type. I have
questions regarding the SDP mapping process.

Pages 21 and 22 in the SDP specification (RFC 2327) indicate the
following mechanism to map a dynamic format ID :
 
m=<media> <port>/<number of ports> <transport> <fmt list>
 
and 
 
a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
parameters>]

Therefore,  an SDP description ought to be able to incorporate the
following two lines:
 
m=audio 49232 RTP/AVP 98
a=rtpmap:98 <AC3_ENCODING_NAME>/48000
 
How does the AVT group suggest I define <AC3_ENCODING_NAME>? Should I
submit a new Internet Draft describing a full profile specifically for
this format, or can I simply use the AC-3 RTP Payload document itself as
the vehicle that registers a valid AV Profile encoding name?
 
Alternatively, though the current AC-3 RTP Payload draft document does
not currrently define an AV RTP Profile encoding name, it does define a
MIME type description for AC-3. The SDP spec allows MIME types to
describe media encodings, but the wording of the SDP spec (page 22)
implies that this is only intended to be used for non-RTP streams. 
 
Which is the most appropriate course of action:
 
1) Define a new RTP profile, 
2) Use the AC-3 RTP payload document to define a valid encoding name for
the AC-3 format, or
3) Use the defined MIME type to map a dynamic AV Profile payload type to
AC-3?

Thanks in advance,
Todd Hager 
Dolby Laboratories
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