[xiph-rtp] header caching and chaining

[Aaron Colwell]

On Fri, Feb 04, 2005 at 08:29:07PM +0000, Ram?n Garc?a wrote:
> There is a strong problem with supporting chaining: it breaks encapsulation. 

How so? In the sane case (ie number of logical stream don't change) all 
chaining does is allow you to change the codec parameters in the middle of
playback.

> 
> 
> Single stream                   |  Several Streams
> --------------------------------------------------------------------
> Vorbis streams                 | OGG Container
> Different RTP sesssions    | RTSP 
> 
> 
> Aaraon, why are you saying that not supporting chaining would imply
> not serving some OGG files. Don't RealMedia, Quicktime and other
> formats use different RTP sessions for each stream contained?

Yes they do. The problem is that there is a subset of valid ogg files that
allow the number of streams to change throughout the duration of the clip. For
on-demand clips you can figure out how many logical streams at SDP generation
time. For a live or simulated live session that does chaining, you don't have
enough info at SDP generation time to generate the right number of streams. 
The only way to make it work inside the framework is to describe 1 stream in
the SDP and specify rtpmap info for both Vorbis and Theora. You can then use
SSRC multiplexing on that single RTP channel to keep the streams seperate.

> 
> On the other hand, there is an important argument in favour of
> supporing chaining: allowing dynamic changes as a response to changes
> in the network conditions. So I think that limited support for this
> case would be justified.

Right. That is one reason. 

> 
> Aaron, how does RealAudio handle changes in network bandwidth?
> Different RTP sessions, or just one?

The different bitrates are multiplexed on the same RTP channel. We basically
have substream IDs in the packets. Audio and video are on seperate RTP 
channels.

Aaron

> _______________________________________________
> xiph-rtp mailing list
> xiph-rtp at xiph.org
> http://lists.xiph.org/mailman/listinfo/xiph-rtp
>