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

[Colin Perkins <csp@csperkins.org>]

--> "Hager, Todd" writes:
>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.

Each application using the AC-3 payload format picks a payload type number
from the dynamic range each time it starts.  The chosen value is exchanged
in SDP, as part of the call setup signalling (e.g. using SIP or RTSP).

>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

Correct. The actual payload type number that is assigned will be different
each time an application is started, and the "a=rtpmap:" line is used to
convey the mapping from the fixed encoding name to the ephemeral payload
type number.

>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. 

RTP originally used a separate namespace for payload format names, but 
now uses the MIME namespace.  The registration procedure is defined in 
draft-ietf-avt-rtp-mime-06.txt.

>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?

The third option is correct. The MIME type defined in the AC-3 payload
format is used as the SDP encoding name (The MIME type registration in
Section 4 of draft-flaks-avt-rtp-ac3-03.txt looks good, I don't think
you need to add anything).

Cheers,
Colin
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