Re: [rohc] ROHCv2: co_repair revisited & coverage

[Carl Knutsson <carl.knutsson at effnet.com>]

Hi Kristofer,

I just want to point out that all not all control fields in dynamic
chain are covered by a crc for this packet. It differs from how these
fields are handled in co_common and IR.

- In co_common, they are protected by the crc verification because
depending fields are covered by the crc.
- In IR the control fields itself is covered by crc-8.

In this case since there are no crc8 over the field itself and depending
fields are sent irregular(uncompressed). So decompression may be
successful even with a broken decompressor context. The fields
'checksum_coverage' and 'ip_id_behavior' in outer header comes to mind.
I am not sure this will be a problem, in this case the next packet will
fail. Just keep it in mind, so that we don't make the same misstakes as
with the IR-DYN in rfc3095.

/Carl Knutsson

Kristofer Sandlund wrote:
> Hi all,
>
> we're finishing up the next revision of ROHCv2, and I
> want it to contain a quite large change to how the co_repair 
> format looks. The more I've worked on the draft, the closer this
> format has become to contining a dynamic chain, so I have decided
> to change this packet format to be
>   <discriminator, CRC-7, CRC-3-control and a dynamic chain>
> instead of its current co_common look-alike format.
>
> This should simplify implementation of these profiles while not
> really changing the protocol very much (also, co_common becomes
> a bit simpler since we can get rid of the repair_flag from it).
>
> If you wish to object, let me know ASAP before we submit the new
> revision (of course, it can be reverted for a later rev. of the
> draft if the WG objects, but it would be easier to do so now if
> someone has reasons to object).
>
> /Kristofer
>
>
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>   


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