.MP4 File Format – Box structure

MPEG-4 Part 14

An MP4 file is built from the ISO base media file format, ISO/IEC 14496-12, which ISO sells and this reference could not read. Layouts on these pages come from memory of the standard and from public implementations, with the box names checked against the MP4 Registration Authority; treat the byte offsets as a guide. All numbers are big-endian.

Box header

OffsetSizeTypeNameMeaning
04uint32sizeTotal size of the box, including this header. 1 means a 64-bit size follows the type; 0 means the box runs to the end of the file.
44FourCCtypeThe box type. uuid means a 16-byte extended type follows.
88uint64largesizePresent when size is 1.
…16UUIDusertypePresent when type is uuid.

A full box adds a one-byte version and a 3-byte flags after the header. Most boxes that have fields are full boxes.

ftyp

The file type box comes first in the file. A segment instead starts with styp, which has the same layout.

OffsetSizeNameMeaning
04major_brandThe brand the file is best read as.
44minor_versionVersion of the major brand, informative.
84 × ncompatible_brandsBrands the file conforms to, to the end of the box.

Other top-level boxes

BoxMeaning
mdatMedia data: the encoded samples. Its contents are found through the sample tables, not by reading the box itself.
free, skipFree space, ignored.
uuidA box whose type is its 16-byte UUID.
pdinProgressive download information.
moof, mfra, styp, sidx, ssix, prft, emsgSee Fragments.
metaTimed and static metadata; a full box holding hdlr and item boxes.

The box tree

  1. ftypfile type
  2. moovmovie
    1. mvhdmovie header
    2. trakone per track
      1. tkhdtrack header
      2. edtswith elst, the edit list
      3. treftrack references
      4. mdiamedia
        1. mdhdmedia header
        2. hdlrhandler
        3. minfmedia information
          1. vmhd, smhd, hmhd, nmhdmedia header by kind
          2. dinfwith dref, data references
          3. stblsample tables
    3. mvexpresent when the file is fragmented
    4. udtauser data
  3. mdatthe media
The usual boxes of an MP4 file and where they nest.