[vorbis-dev] Update on Vorbis RTP I-D

Michael Smith msmith at xiph.org
Tue Feb 11 04:14:51 PST 2003

On Tuesday 11 February 2003 22:47, Phil Kerr wrote:
> Hi all,
> I'm in the final stages of putting a new Vorbis RTP draft together, the
> new sections are below.
> There are a number of open questions:
> 1.) At present there is a 16 bit length field for the codebooks.  As
> they are spec'd as being unbound in length, but typically around 15K,
> are there situations where they may be greater than 64KB?  The size
> limit can be extended to 2MB as there are 5 bits unused.

They're actually typically 4 kB (with 1.0 encoders), but they can be much 
larger. Segher has an experimental tool which can generate several megabyte 
long headers.

However, I think it'd be entirely reasonable (and in fact a very good idea) to 
make the RTP/vorbis spec limit this. 64 kB is one possibility, shrinking it 
substantially might also be reasonable. 

> 2.) We discussed using the RTP seq ids to keep the config RTCP messages
> in sync with the RTP data stream.  I've changed this slightly to use
> timestamp values.
> 3.) Codebook delivery has a number of different ways.  I'm currently
> looking into TCP over RTP.

Codebook delivery is, of course, the largest problem in terms of practical 
implementations. Perhaps have multiple methods with flag(s) to indicate which 
are available for a specific stream?

> 4.) There is now an overflow flag.  This is used if the size of the
> codebooks and/or the comment headers is larger than the max RTCP packet
> (64k).  If this flag is set clients must obtain the headers from the URI
> specified in the overflow URI field or SDP value.  This will add a small
> amount of complexity to the client.

Neccesary? I'd suggest mandating codebook and comment headers of < 64 kB. 
There are no good reasons for going beyond that in a streaming context.

> 4.) Port numbers.  Should we reserve fixed ports for servers with the
> IANA?  1190/91 are unused.

Is there a need to? I would think (but I'm not an expert in the area) that the 
content (vorbis) should be sufficiently independent of the delivery mechanism 
such that a reserved port number is irrelevant.

> 5.) The MIME type is audio/vorbis
> I'll have a final draft ready in the next day or so and if possible I
> want to submit the update to the IETF by next Monday so there is
> discussion time before their next meeting.
> Feedback and comments welcomed.
> Regards
> Phil


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