[xiph-rtp] about theora-over-rtp draft

[Ralph Giles]

On Fri, Jul 21, 2006 at 12:08:33PM +0200, Simon Morlat wrote:

> If this is a known fact that the header is fixed size, then it should be written in the rfc.
> However I don't think it's a good idea, this may prevent theora developers to make evolutions to the codec.

I agree with both of these points, especially because the implicit 
length isn't a limitation of the ogg embedding. If we did change
the length of the first header in a future rev of theora, we'd
almost certainly also bump the version number, so you could switch
on that, but it's still a constraint we'd have to remember.

Perhaps more importantly, the current theora spec allows additional
header packets if they are optional for decode (custom colourspace
definitions, application-private data and so on). Having these would
also break the 'only one length field' hack.

So we should either go to 'length field for every packet in the packed 
header format' or we need language about how such extra headers cannot
be transmitted this way, and to define the length of the identification 
header".

Apparently I'm being careful not to express a decision here. :)

Simon, as regards the fragmentation bits, this was designed for vorbis, 
where it's helpful to know you have a truncated packet because the 
decoder can handle that for partial decode. You can infer this just 
as well from your marker bit scheme, but I think there's more value in 
having a consistent scheme between the codecs.

One advantage here would be that if we can use the marker bit for 
fragmentation, we could drop the fragmentation *and* packet type
fields and only go back to a 32 bit configuration id. (or better yet,
drop that too! :-) That would be a serious simplification, perhaps worth 
losing the fragmented/packed distinction.

Anyway, I want to say thanks to you too, having feedback from actual 
implementation is invaluable.

Cheers,
 -r