Re: [AVT] Update (-09) of Timed Tex draft (WAS: RE:Commentsondraft-ietf-avt-rtp-3gpp-timed-text-08.txt)

[Colin Perkins <csp at csperkins.org>]

Jose,

On 11 Feb 2005, at 14:31, Rey Jose wrote:
> Jan and I have discussed this offline and came with the following 
> proposal, which is much simple and requires almost no changes. It 
> doesn't mess up the timestamp while still associating a TS to SDs (in 
> some way). As follows:
>
> 1.- if a text sample needs an SD SHOULD include it always *before* the 
> text samples it applies to.
> 2.- SD become available for use as indicated by the TS in the packet 
> so even if more than one (SD,TextSample) pair is present in the packet 
> this is valid since they are timely ordered. If some SDs are available 
> 'too early', so what?
> 3.- if you send spare SDs nothing changes, only there are no text 
> samples
> 4.- The SHOULD in 1) is not a MUST because there may be cases you can 
> relax this, e.g., SDs can be omitted if the server knows that these 
> are available at the client. This allows applications that can 'live' 
> with some SD losses and still display something useful

This, and the text that is in -13, seem to be unnecessarily complex. We 
talked about changing:

        TYPE 5 units are exception: TYPE 5 units do not make use of the
        timestamp, but instead become active upon reception and not at
        the time instant indicated by the timestamp.  Therefore, if RTP
        packets contain only one TYPE 5 unit or only (several) TYPE 5
        unit(s), the RTP timestamp SHALL be set to the current value of
        RTP timestamp plus one.  Adding one (1) unit, as opposed to just
        using the current timestamp, further allows the use of the
        timestamp for identifying RTP packets that carry fragments of
        the same text sample.

into something like:

        TYPE 5 units are exception: TYPE 5 units can be used to convey
        sample descriptions that are used by many samples. Accordingly,
        their timestamp does not represent the instant when they are
        played out, but instead the instant at which they become 
available
        for use. A sample description conveyed in a TYPE 5 packet MUST 
NOT
        be referenced by a unit with earlier timestamp.

which to me seems much clearer than the text in section 4.5 of draft 
-13. I don't understand the objections raised about sending duplicate 
type 5 data: surely if you give such duplicate data the timestamp of 
the packet in which it's sent, you get exactly the same semantics as in 
the earlier draft, while retaining the RTP timing model and accurate 
jitter calculations?

Colin


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