Re: [Cellar] Zaheduzzaman Sarker's No Objection on draft-ietf-cellar-matroska-17: (with COMMENT)

[Steve Lhomme <[e-mail]>]

Hi,

Comments below:

> On 4 Jun 2023, at 20:55, Zaheduzzaman Sarker via Datatracker <[e-mail]> wrote:
> 
> Zaheduzzaman Sarker has entered the following ballot position for
> draft-ietf-cellar-matroska-17: No Objection
> 
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> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> Thanks for addressing my discuss points.
> 
> Copying the discuss points below -
> ======================================================================================================================
> However, I have two observation/questions that I would like to discuss to have
> clear opinions and better understanding.
> 
>    - Top-Level Elements are optional fields in the segment. While segment is a
>    MUST part in the container but segments values (elements) are MAY, this to
>    me says one can just put a dummy segment in the container and it will be
>    fine. is that correct interpretation? however, there is a RECOMMENDED
>    segment element, so how should we interpret the statement that Top-level
>    Elements are all MAY?
> 
>    - Even if the Top-Level Elements MAY be available in the container, some of
>    the elements has MUST parts when they are present. However, I have not
>    notices description of the consequences or error handling is those MUST
>    parts are not available in the elements. I wonder what would be the course
>    of action if the MUST parts of a certain element is not there. In general,
>    I was expecting error handling in this specification which is not there and
>    would like to discuss the reasoning behind it.

We don’t tell how to handle errors because it’s really up to the app/library to make these decisions based on the goal of the software. There already exist plenty of existing implementations that handle each error differently. We can tell people what they should and should not do but in the end we can’t make these kind of decisions for them. For example a streaming may just skip bogus data and some other will just decide to stop playback because an error occurred. There’s also a huge number of ways things could go wrong. It wouldn’t be possible to cover everything, so developers would still have to decide for themselves anyway.

So going back to your previous question, an (almost) empty Segment is valid. But in the end it’s useless. It may be created and spread by accident. But in the end it’s harmless. There’s no content to be tried to be played. However there is one Top-Level Element that is mandatory: the Info Element which gives a short description of what is in the Segment. There’s a duration which cannot be 0 if it’s set. In that case there must some content to play (Tracks + Cluster) element, otherwise the element would be lying. If there’s no duration, it’s possible to have links to other segment (within the Info Element or via Chapters).

There are 2 elements that are mandatory in the Info Element: Muxing App and Writing App. That’s the minimum you can have in a Segment. Any reader should not expect other elements to be there and act accordingly. There’s probably a bunch of implementations that would not like a Matroska file without a Track description. But the element was never mandatory so they did it wrong. Just like some reader expect certain sizes for the EBML header. These are bogus implementations that work most of the time.

I hope this clarifies things for you.
Steve

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