Re: [Cellar] On the multiplicity of Info elements

["Sebastian G. <bastik>" <[e-mail]>]

12.01.2016, 14:36 Moritz Bunkus:
> Hey,
> 
>> From a sample set of 59,881 Matroska files uploaded to archive.org
>> only 119 of them include CRC-32 Elements. Almost 0.2%! Additionally
>> the webm Document Type lists the CRC-32 Element as unsupported. So I
>> think it’s true that it’s rarely bothered with. The implementation of
>> CRC-32 Elements does add many complications and perhaps demand has
>> been too low to resolve them. Moritz may have comments here.
> 
> That's pretty much spot on. The CRC32 element was added comparatively
> late; a lot of muxers had already been created by that time.
> 
> Most requests I've received where more concerned with error recovery
> than with error detection. This is something that CRC32 wasn't meant
> for; additionally CRC32 elements weren't meant to be written in the
> cluster elements which make up the bulk of a Matroska file's data.
> 
> Yes, there is a feature request[1] for adding support in mkvmerge and
> mkvinfo. It has been created in 2010, but except for those two people
> mentioned in the bug I haven't really received any other requests for
> such a functionality.
> 
> Kind regards,
> mosu
> 
> [1] https://github.com/mbunkus/mkvtoolnix/issues/543
> 

Hi,

to said ticket you added two entries [2][3] which mention a problem with
verifying checksums.

Would that be much easier to handle if the CRC would be about the data
that should be present in the file, but are omitted due to default
values aren't written? Compared to creating a checksum of the data that
actually get written and verifying said checksum.

I had a rather strong opinion towards including a CRC that covers the
elements that are written. I'm just curious and if something can be
solved elegantly rather than having to invest too much resources into
it, it might be worth reconsidering. After all it would be defined
within a proper specification.

Back then when I offered the bounty I expected matroska to be used a lot
more often in the future by other people than me. My usecase for
checksums was indeed just error recovery. However I am not against
extending the potential usecases to have applications use it to detect
errors.

[2] https://github.com/mbunkus/mkvtoolnix/issues/543#issuecomment-171036571
[3] https://github.com/mbunkus/mkvtoolnix/issues/543#issuecomment-171422587

--
Sebastian