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>]

On 8 Feb 2005, at 08:48, Magnus Westerlund wrote:
> I am in support of this change, however as outline below I don't think 
> we have thought yet of all the issues.
>
> Rey Jose wrote:
> > Jan,
> >  this would simply translate into requiring that the SD is assigned 
> > the timestamp of the RTP packet in which is contained. That is:
> > "TYPE 5 units are an exception: TYPE 5 units receive their timestamp 
> > from the RTP packet in which they are contained.  These 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."
> I don't think it will be this simple. We must consider how we assign 
> timestamps to SDs and when packet losses occurs. Let me give an 
> example:
>
> 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
>
> IF you in the above case would loose packet 1, you can only utilize 
> packet 2 if you get packet 3 and its SD.

This is the behaviour in the -09 draft, yes.

> 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.

Colin


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