[xiph-rtp] header ident decision

[Ralph Giles]

On Sun, May 01, 2005 at 09:52:56AM -0700, Ralph Giles wrote:

> One this I think we can define is in-band transmission. Just send the
> header packets in the RTP stream like they were any other type. The 
> 16 bit setup ident field of the header packets must match the setup 
> ident of the data packets that rely on them. In the default case the 
> three standard headers are sent at the beginning of the stream, just
> as they are in Ogg. The server MAY also retransmit the headers 
> periodically; clients MUST handle such header packets when they occur.

Some further clarification from discussion with Aaron on IRC:

Clients MUST handle any header packet at any point in the RTP stream; 
there is no restriction on ordering or grouping. When using in-band 
transmission, the server SHOULD send the initial 3 header packets, in 
order, at the beginning of the stream, including the comment header even 
if it contains only the required vendor string from the encoder.

The server MUST ensure that the setup ident field in the payload header 
attached to any ident or setup header packets is a consistent and unique 
mapping within an RTP session, so that clients can safely discard such 
packets if they already have a decode setup for that ident field.

The server MAY choose to send multiple comment packets with the same 
ident field value. Such packets, when received, indicate to the client 
that the new set of metadata applies to the audio data that follows. In 
this way song title changes and so on can be indicated without the 
overhead of a chain boundary. and sending a completely new set of 
headers. Applications spooling an RTP stream to an Ogg stream or file 
should insert a chain boundary or begin a new file when these 
metadata updates occur, even though the decoder setup has not changed.

 -r