RE: [rohc] Interpretation intervals for TS encoding

["Ghyslain Pelletier \(LU/EAB\)" <[e-mail]>]

Hi Endre,

It strikes me that the text that you are addressing in
draft-ietf-rohc-rtp-impl-guide-19.txt is rather old, it probably comes
from a version prior to this draft becoming a WG draft. Therefore I
think it is sound to revisit it.

I think that this sums up to answering to the following:

1) When does the HD start using TB decoding
2) What happens to the interpretation interval during
   transitions between Scaled TS encoding and TB encoding?
   (i.e. does the HC have to send more bit or some reason?)

My understanding is that if the answer to 1) is clear, than 2) is
irrelevant as, as you wrote yourself, the compressor will use a
reference that it has confidence that the decompressor has, and it will
use the correct interpretation interval as per section 4.3.

The answer to 1) is also clear, as you wrote yourself again, i.e. the
decompressor uses TB decoding as soon as it sees TIME_STRIDE>0, as per
section 5.7:

   TS: The compressed RTP Timestamp value.

      If value(TIME_STRIDE) > 0, timer-based compression of the RTP
      Timestamp is used (see section 4.5.4).

So, as a result of your comment, my proposal is to remove the last
paragraph in the text of section 4.3, i.e.:

   Since two different p-values are used, the compressor must take this
   into account throughout the process of enabling timer-based
   compression (see section 4.8 of this document). During transition
   from window-based compression to timer-based compression, it is thus
   necessary that the compressor keep k large enough to cover both
   interpretation intervals.

as it is not relevant (besides that it was in itself rather unclear in
its own magical ways!).

Thanks

/Ghyslain

Endre Szalai (IJ/ETH) wrote:
> Hi ROHCers,
> 
> reading the "Corrections and Clarifications to RFC 3095"
> (draft-ietf-rohc-rtp-impl-guide-19.txt) document, the last paragraph 
> of section 4.3 is not really clear for me.
> 
> "Since two different p-values are used, the compressor must take this

> into account throughout the process of enabling timer-based  
> compression (see section 4.8 of this document).
> During transition  from window-based compression to timer-based 
> compression, it is thus  necessary that the compressor keep k large 
> enough to cover both  interpretation intervals."
> 
> When the HC wants to use timer based compression, it will start 
> sending a positive TIME_STRIDE value (after of course a valid CLOCK 
> option is propagated from the HD). If the HD receives such a packet, 
> it will switch to timer based compression immediately. This means, 
> that the HD will use the value for "p" as specified in section 4.5.4 
> in RFC 3095) when decompressing received TS bits (if any), and not the

> "p"
> value for the W-LSB encoding.
> 
> Why and how the HC should consider the "p" value for W-LSB encoding in

> this case ?
> 
> The HC keeps track of all possible references the HD may have, and the

> HC will simply apply timer based compression with the proper "p" 
> value. Since HD will use the same "p" value (TIME_STRIDE > 0), how 
> could the 2 different interpretation intervals interfere ?
> 
> Could someone give a clarification on this (maybe an example) ?
> 
> Thanks,
> Endre
> 
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