Re: [rohc] Clarification required for CSRC List Compression

[Klaus Warnke <klaus.warnke at acticom.de>]

Anil, 
find my respones inline.

At Wed, 14 Oct 2009 10:06:42 +0530,
Anil Maguluri wrote:
> Hi All, 
> 
> I have few queries regarding the CSRC List Compression. 
> 
> Please clarify me whether my understanding is right or wrong. 
> 
> Scenario1: The reference list is set b/w compressor and
> decompressor.  After that compressor received RTP packet with the
> same CSRC list [list is not changed].
> 
> Q1. How compressor will inform to decompressor about the CSRC List?
> 
> As per my understanding, Compressor will inform [send Encoding Type
> 0/1/2/3] to decompressor only when the list changes. In the
> scenario1, compressor will not send any information about the CSRC
> list to the decompressor. Decompressor will reconstruct the packet
> using the ref_list.

That is correct.  

If the compressor is confident, that the de-compressor knows the CSRC
list (after repeating x-times or receiving a ACK) there is no need to
send it again.

> Scenario2: The reference list is set b/w compressor and
> decompressor.  After that compressor received rtp packet with CC
> value is zero.
>
> Q: How will compressor will inform to decompressor about the CSRC
> List?
> 
> As per my understanding, Compressor will always compare with
> ref_list to create the compressed packet. compressor will generate
> new Gen_ID and it compare with the ref_list to create compressed
> packet. In the above scenario, compressor will send removal scheme
> only using the removal bitmask setting the all 1's [based on the
> ref_list items]. is it right?  Otherwise decompressor will
> reconstruct the packet using the ref_list.

This is right.
To use the removal scheme is maybe not the best choice. We are using
the generic scheme, because it is smaller. 

> Scenario3: The reference list is set b/w compressor and
> decompressor.  Compressor moved to IR->FO state. Now compressor
> received the RTP packet with the same CSRC list [List is not
> changed]. Compressor selected IR-DYN packet based on the state and
> mode.
> 
> Q: What value to encode in CC field and Generic CSRC List field?

There is nothing special in this case.
You send the same dynamic chain as used in the inital IR packets.
The IR-DYN packet updates the dynamic values of the context and no
part can be omitted. See 3095 page 99.

> Thanks & Regards, 
> Anil Kumar Maguluri 

br
Klaus