[Matroska-devel] clarifications on the use of Unknown Sized Elements

Moritz Bunkus moritz at bunkus.org
Tue Jun 9 08:48:23 CEST 2015


Hey,

> - Only Elements that are Master-elements may use an unknown size.

I would be good with this, even though I don't know whether or not we
had that restriction before.

> - When is the Element of Unknown Size ended. In discussions (in
>   github, mailing list, and irc) there has been two answers to this:
> 	1. Upon the first occurrence of an Element that is not a valid
> 	   sub-element of the unknown-sized element.
> 	2. Upon the first occurrence of an Element that is at the same or a
> 	   higher level than the one with an infite size

I was the one who said 2.; basically what I meant was 1. 1. is more
precise than my wording yesterday (2.), therefore disregard 2., please.

1. also hints at the problem with elements than can occur at multiple
levels (EbmlVoid, SimpleTags).

> - Dependency on Schema

> For the SimpleTag option available, the interpretation of the ending
> of the element presumes that the parse has knowledge of the EBML
> Schema/Profile of mastroska in any version 1-4. I'd like to include
> some statement to say that the use of an element of unknown size
> requires the use of the corresponding schema.

Hmm… going with 1. from before we could indeed have a scenario where an
element from (a hypothetical version) v5 may include an element from
earlier versions, e.g. we introduce KaxÜberTags (v5 element) which may
include the already existing KaxTag elements (v4).

In that situation a parser that only knows v1-v4 but not v5 could find
itself in a situation where a KaxÜberTags master has an infinite size,
but the parser would then erroneously see that KaxTag as the end of the
unknown infinite-sized KaxÜberTags.

So you may be right, and Steve might be wrong.

Kind regards,
mosu
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