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

[Colin Perkins <csp at csperkins.org>]

Magnus,

On 13 Apr 2005, at 12:40, Magnus Westerlund wrote:
> Colin Perkins wrote:
> > Packet 0: Type 1: SIDX=1
> > Packet 1: Type 5: SIDX=2
> >           Type 1: SIDX=2
> > Packet 2: Type 1: SIDX=2
> > Packet 3: Type 5: SIDX=2 (Repetition)
> >           Type 1: SIDX=2
>
> > > With the proposed timestamp model, the timestamp of the TYPE 5 unit 
> > > in packet 3 should have a TS equal to the TS of packet 1, rather 
> > > than the TS of packet 3 to indicate that it is useful from the TS 
> > > time of packet 1.
> > Why? If you give the type 5 unit sent in packet 3 the timestamp of 
> > packet 3, then the behaviour is unchanged from the previous, except 
> > that the timing model is used to decode the data.
>
> The reason is that otherwise, you would not care to depacketize packet 
> 3 until after you have tried to consume packet 2 and failing that due 
> to lack of sample description, and moved on.

Yes, of course. That's exactly the behaviour that was defined in the 
earlier versions of the draft, which made the Type 5 units available 
for use as soon as they arrived.

The changes in the latest versions of the draft add a lot of complexity 
for very minimal benefit (i.e. they help if you have packet loss in 
between two repetitions of a type 5 unit and a playout buffer long 
enough such that the repeated copy of the unit arrives before the data 
referencing the first has been played out). I just don't think the 
added complexity is worth it for this use case.

Colin


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