[Libav-user] How to determine duration in fragmented mp4 init segment

[Jean-Yves Avenard]

Hi there.

I'm asking here now as I've been banging my head on this for a while
now and can't find a solution.
Seeing that the file I'm analysing was creating using libavformat ;
I'm hoping someone here would know...

So here I have an init segment:
http://people.mozilla.org/~jyavenard/mediatest/fragmented/facebook-paper_dash.mp4

Now, all movie headers (mvhd atom), media header (mdhd) or track
headers (tkhd atoms) indicate that the duration is 0.

(can check with this page:
http://people.mozilla.org/~jyavenard/tests/mse_mp4/paper-init.html)

Yet, IE11 and Chrome reads the duration as 120.187s which makes sense
as the original mp4 I extracted it from was 2 minutes long.

Here is the details of this mp4 init segment:

        [ftyp] size=8+28
          major_brand = isom
          minor_version = 200
          compatible_brand = isom
          compatible_brand = iso2
          compatible_brand = avc1
          compatible_brand = mp41
          compatible_brand = dash
        [free] size=8+47
        [moov] size=8+1168
          [mvhd] size=12+96
            timescale = 1000
            duration = 0
            duration(ms) = 0
          [mvex] size=8+64
            [trex] size=12+20
              track id = 1
              default sample description index = 1
              default sample duration = 1001
              default sample size = 0
              default sample flags = 10000
            [trex] size=12+20
              track id = 2
              default sample description index = 1
              default sample duration = 1024
              default sample size = 0
              default sample flags = 0
          [trak] size=8+467
            [tkhd] size=12+80, flags=f
              enabled = 1
              id = 1
              duration = 0
              width = 1920.000000
              height = 1080.000000
            [mdia] size=8+367
              [mdhd] size=12+20
                timescale = 30000
                duration = 0
                duration(ms) = 0
                language = eng
              [hdlr] size=12+33
                handler_type = vide
                handler_name = VideoHandler
              [minf] size=8+282
                [vmhd] size=12+8, flags=1
                  graphics_mode = 0
                  op_color = 0000,0000,0000
                [dinf] size=8+28
                  [dref] size=12+16
                    [url ] size=12+0, flags=1
                      location = [local to file]
                [stbl] size=8+218
                  [stsd] size=12+138
                    entry-count = 1
                    [avc1] size=8+126
                      data_reference_index = 1
                      width = 1920
                      height = 1080
                      compressor =
                      [avcC] size=8+40
                        Configuration Version = 1
                        Profile = Main
                        Profile Compatibility = 40
                        Level = 31
                        NALU Length Size = 4
                        Sequence Parameter = [67 4d 40 1f ec c0 3c 01
13 f2 e0 22 00 00 07 d2 00 01 d4 c0 1e 30 63 34]
                        Picture Parameter = [68 e9 78 f2 c8]
                  [stts] size=12+4
                    entry_count = 0
                  [stsc] size=12+4
                    entry_count = 0
                  [stsz] size=12+8
                    sample_size = 0
                    sample_count = 0
                  [stco] size=12+4
                    entry_count = 0
          [trak] size=8+407
            [tkhd] size=12+80, flags=f
              enabled = 1
              id = 2
              duration = 0
              width = 0.000000
              height = 0.000000
            [mdia] size=8+307
              [mdhd] size=12+20
                timescale = 48000
                duration = 0
                duration(ms) = 0
                language = eng
              [hdlr] size=12+33
                handler_type = soun
                handler_name = SoundHandler
              [minf] size=8+222
                [smhd] size=12+4
                  balance = 0
                [dinf] size=8+28
                  [dref] size=12+16
                    [url ] size=12+0, flags=1
                      location = [local to file]
                [stbl] size=8+162
                  [stsd] size=12+82
                    entry-count = 1
                    [mp4a] size=8+70
                      data_reference_index = 1
                      channel_count = 2
                      sample_size = 16
                      sample_rate = 48000
                      [esds] size=12+30
                        [ESDescriptor] size=2+28
                          es_id = 2
                          stream_priority = 0
                          [DecoderConfig] size=2+20
                            stream_type = 5
                            object_type = 64
                            up_stream = 0
                            buffer_size = 0
                            max_bitrate = 192082
                            avg_bitrate = 192082
                            DecoderSpecificInfo = 11 90 56 e5 00
                          [Descriptor:06] size=2+1
                  [stts] size=12+4
                    entry_count = 0
                  [stsc] size=12+4
                    entry_count = 0
                  [stsz] size=12+8
                    sample_size = 0
                    sample_count = 0
                  [stco] size=12+4
                    entry_count = 0
          [udta] size=8+90
            [meta] size=12+78
              [hdlr] size=12+21
                handler_type = mdir
                handler_name =
              [ilst] size=8+37
                [.too] size=8+29
                  [data] size=8+21
                    type = 1
                    lang = 0
                    value = Lavf55.12.100

I can't for the life of me figure out where it's getting that 120.187s
duration...

Any ideas?
thanks