RE: [rohc] Interpretation intervals for TS encoding

Ghyslain, Endre, others,

I fully agree with your conclusions.

This paragraph originates from the initial individual version
of this draft, dated November 2001. It had its own section
called "4.2.  Transition to timer-based compression", but the
text was later incorporated into the section about
interpretation intervals (added in rev 02, Oct 2002).

When now looking into this again, I agree with the conclusion
that 1) is clear from section 5.7 of RFC 3095. It is a mystery
to me how and why anyone would interpret this differently.

>From the mail archive we can find some discussions with
interpretations in line with the questioned paragraph in the
implementer's guide, but without arguments *why* that
should be needed (see thread titled "What is p value for TS?"
from September-October 2002).

So, I agree with the suggestion to remove this paragraph from
section 4.3 of the current draft. Since I have not seen any
objections, I will do this change to the draft right now.

/L-E

> 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 bits for 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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