Subject: [rohc] Hi , everyone when the compressor update it`s compression context?

From: "wanxiujin" <[email-masked]>
Date: Mon, 27 Feb 2006 15:47:56 +0800 (CST)

Hi everyone：
  
  I have a question about ROHC to ask you. In O/R mode,we let the state transtate from low state to high state when the compressor received a Ack packet.But when should the compressor  update it`s context?
  For example: if working in O mode FO state, the compressor send IR-DYN packet to the decompressor.The decompressor correctly decompress the IR-DYN packet into the IP packet and the decompressor will update the decompression context ,and the decompressor will send a Ack packet to the compressor.  So we can find that the decompressor update it`s context once it decompresses the IR-dyn packet correctly .But when the compressor update the context? if at the time when the compressor receive the ACK packet,the compressor must save all the updating packets to the decompressor  in order to update the context. 
   I don`t known the time when the compressor update the context.Can you tell me??
   Thank you very much.

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From: [email-masked]
Date: Tue, 28 Feb 2006 11:19:51 +0100

Hi Wanxiujin,

Everytime that the compressor sends a packet that will update (parts or all of) 
the context at the decompressor, the compressor updates its own context as 
well -- right away when sending the packet. The compressor thus keeps the 
necessary information related to the packet that was compressed, as reference. 
This information will be used later to compress the fields of the next packet. 
In addition, the compressor normally maintains in the context a history over a 
number of the lastest context-updating packets, possibly as a sliding window of 
items. This information is useful to add robustness (and helps error recovery 
when NACK is received) to the selection of what packet format to use, together 
with the result of the compression. If the packet is not context-updating, then 
the compressor does not update its context (i.e. does keep this header as a 
reference for further compression) and does not keep an item related to that 
header in its context either.

Acknowledgements only helps the compressor to manage this history. For example, 
the history can be managed as a sliding window that can be shrunk (shortened) 
when receiving acknowledgements (i.e. items in the history that are older than 
the acknowledged information can be removed from the history), and that grows 
with one item or each context updating packet that is sent by the compressor.

In your specific example, the compressor updates its context as soon as it 
sends an IR-DYN packet. The acknowledgement that comes later would then help 
your compressor to manage the history of context-updating packets that were 
sent, which is used for robustness and error recovery.

Hope this helps, and I hope that this answers your question.

/Ghyslain

Quoting wanxiujin <[email-masked]>:

> 
> Hi everyone£º
>   
>   I have a question about ROHC to ask you. In O/R mode,we let the state
> transtate from low state to high state when the compressor received a Ack
> packet.But when should the compressor  update it`s context?
>   For example: if working in O mode FO state, the compressor send IR-DYN
> packet to the decompressor.The decompressor correctly decompress the IR-DYN
> packet into the IP packet and the decompressor will update the decompression
> context ,and the decompressor will send a Ack packet to the compressor.  So
> we can find that the decompressor update it`s context once it decompresses
> the IR-dyn packet correctly .But when the compressor update the context? if
> at the time when the compressor receive the ACK packet,the compressor must
> save all the updating packets to the decompressor  in order to update the
> context.