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

["Jose Rey" <Jose.Rey at eu.panasonic.com>]

Colin,

once it is agreed (?) to change the timestamp rules to the easier option: where all sample descriptions are placed at the beginning before ocurrence of any non-TYPE5 units, the answer to the 2nd part of the email follows:

> You seem to be 
> suggested that they can be used before they're transmitted?
> 
> > On this regard, this discussion in the intro of section 4 
> was moved to 
> > 4.5, where the following was added:
> >
> >      Regarding sample descriptions in aggregate payloads: 
> since TYPE 5
> >      units may potentially apply to several units in the stream, a
> >      sender shall ensure that a copy of the sample description is
> >      received before the affected text sample is due.  Therefore, a
> >      copy of the sample description SHOULD be placed in the payload
> >      before the text sample it applies to.  Of course, if 
> several text
> >      samples in a payload use the same sample description, once per
> >      payload is enough.  A sender MAY choose to omit the sample
> >      description if it knows by some other means, such as payload
> >      specific feedback messages [21], that the sample 
> description has
> >      arrived at the receiver or if it employs additional transport
> >      resiliency measures (Section 5), for example.
> 
> I would suggest changing this to:
> 
> 	Correct reception of TYPE 5 units is important since 
> their contents
> 	may be referenced by several other units in the stream. 
> Accordingly,
> 	they MUST be given at timestamp not later than that 
> given to those
> 	other units. It may be desirable to resend the sample 
> descriptions
> 	periodically, or to use other transport resiliency mechanisms.
> 
> since the text in the draft implies that a copy of the sample 
> description is sent in every packet, rather than allowing it 
> to be sent once and referenced by later packets.
> 

Since it is the codec that shall buffer the SDs, then it is possible in some cases to 'skip' putting SDs in every single payload and be (or just relatively) confident that the units will be correctly displayed: for cases such as reliable channels or when the server knows the SD was received by other means; hence, I thought of a SHOULD and not a MUST. Would this text be fine?

--
Correct reception of TYPE 5 units is important since their contents may be referenced by several other units in the stream.  Sample descriptions MUST be placed in the aggregate payload before the occurrence of any non-TYPE 5 units and MUST receive the RTP timestamp of that packet.  Additionally, it is RECOMMENDED to include a copy of the sample description if referencing text samples are present.  This is because the decoder is, in general, not able of using the text sample if the sample description is missing.  Of course, if several text samples in a payload use the same sample description, once per payload is enough.  
Note that sending a sample description together with every text sample (or set of samples) may result in a situation where the same sample description is present in the receiver buffer many times, but with different timestamp values.  This is a desing choice to ensure correct reception of sample descriptions and may cause moderate overhead.  Nevertheless, a sender MAY choose to omit the sample description if it knows by some other means, such as payload specific feedback messages [21], that the sample description has arrived at the receiver or if it employs additional transport resiliency measures (Section 5), for example.
--

If OK, I will submit a new draft asap,

Cheers,

José



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