[xiph-rtp] Theora "setup idents" and codebook URLs in SDP

[Phil Kerr]

Hi David,

David Barrett wrote:

> So it sounds like the final word is that each Theora data packet will 
> have a 16-bit "setup ident" field (ie, a codebook identifier) in the 
> following payload header:
>
> From: http://lists.xiph.org/pipermail/xiph-rtp/2005-May/000229.html
> >     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
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >    |           Setup Ident         |   Reserved    |C|F|R| # pkts. |
> >    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>
> Currently it sounds like the only requirement is that the "setup 
> ident" be unique within a given session.  Ie, ident #1234 might 
> indicate a different codebook in two different sessions (though it 
> always refers to the same codebook within any given session).  With 
> all this in mind, I have a couple questions:
>
> 1) What is the SDP syntax for associating a particular codebook URL 
> with a given setup ident?
>
> 2) Can I specify many codebooks (each with a different setup ident) in 
> the SDP such that the decoder can "prefetch" the codebooks before they 
> are used in the RTP?

I do not see any problem with this, apart from.....

>
>
> 3) What are the rules for caching codebooks in the decoder?  If setup 
> idents are only unique and persistent within a given session, 
> obviously I can't use those.  I can easily use the URL, but because 
> it's essentially arbitrary (ie, not derived from the codebook itself), 
> it's entirely possible that the codebook "at the other end" of the URL 
> might change without my knowledge (ie, my cached version can become 
> dirty). One proposal is to say:

Without a unique identifier it becomes problematical to cache codebook 
outside a session, unless.....

>
> "A codebook URL SHOULD include a CRC32 of the codebook itself, so as 
> to prevent the codebook referenced by URL from changing, and thereby 
> enabling decoding clients to use the URL as a persistent, 
> globally-unique identifier of the codebook itself, suitable for 
> reliable caching purposes."

Yup.  But how do we prevent clashes with the packet ident?

Regards

Phil

>
> -david
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