Mpeg Dash - fragmentation and segmentation

Question

I am trying to implement Mpeg DASH streaming using shaka packager. To generate segments of duration 5 seconds each, --segment_duration param helps me achieve this.

https://google.github.io/shaka-packager/html/documentation.html#chunking-options

I could see how a fragmented video is represented from the following link

What exactly is Fragmented mp4(fMP4)? How is it different from normal mp4?

1. What is the purpose of fragmentation?
2. Does the packager automatically create fragments when segmented?
3. Does each segment have moof+mdat as represented above?
4. What are subsegments?
5. What happens if --segment_duration and --fragment_duration is set to the same value?
6. What is the purpose of --min-buffer-time?

Answer 1

Segments are a property of DASH. A segment is the minimal download unit.

Fragments are a property of fragmented MP4 files. Typically a fragment consists of moof + mdat.

A fragmented MP4 is usually created as ftyp moov | moof mdat | moof mdat | ... | moof mdat |.

A regular MP4 is ftyp moov mdat or ftyp mdat moov.

A fragmented MP4 is more reliable since individual fragments can be independently decoded. A long lasting recorder is a good use case. In case of a power loss an incomplete fragmented MP4 is stil useful.

In DASH I would align fragments and segments. You probably could have multiple fragments per segments.