MPEG-21 File Format
MPEG doc#: N7925
Date: April 2006
Authors: David Singer (Apple), Mohammed Zubair Visharam (Sony)
Within the ISO/IEC 14496 MPEG-4 standard there are several parts that define file formats for the storage of time-based media (such as audio, video etc.). They are all based and derived from the ISO Base Media File Format , which is a structural, media-independent definition that is also published as part of the JPEG 2000 family of standards. It has a basic box-structuring part, and a definition for timed sequences of multi-media (audio, video etc.) in such a box structured file.
The MPEG-21 File Format  uses the structural definition of a box-structured file as defined in the ISO Base Media File Format, but not the definitions for time-based media. It defines the storage of an MPEG-21 digital item, with some or all of its ancillary data (such as images, movies, or other non-XML data) within the same file.
The family of the storage file formats is based in the concept of box-structured files. Box structured files are used in a number of applications, and it is possible to form ‘multi-purpose’ files which contain the boxes required by more than one specification. Examples include not only the MPEG-21 file format described here, but also formats using the time-structured part of the ISO Base File Format family.
As described above, the general meta-box can be used at the file level to contain a description and its associated or included data. This structure is used for MPEG-21 files. A file-level meta-box is used to hold an MPEG-21 Digital Item Declaration (DID) , The meta-box also contains a list of attached resources; which may have local names, and may be located within the same file or in another file.
The full structural power of the meta-box can be used to give significant flexibility in the structure of the MPEG-21 file.
- Items (other files) required by the digital item may be included in the same file, or in another (possibly combined together in another file). That other file could be another MPEG-21 file, another file from the family, or of another format entirely.
- Items (files) included in this or other files may be fragmented, and those fragments interleaved.
- Items (files) may be protected and the signaling of that protection given.
- Items (files) may be named, to make access of them easier.
There are URL forms defined for the items (files) defined in a meta-box, and these forms allow access to the items from within or outside the file. The general concept is that the MPEG-21 file is a ‘package’ of files that are ‘pre-cached’ at the client, and this cache obviates the need to fetch many individual files when accessing an MPEG-21 digital item.
The brand defined for MPEG-21 files is ‘mp21’, and if used as a major-brand, the matching file extension would be “.m21”. The IANA registry  should be consulted for the appropriate MIME type.
There is a registration authority which registers and documents the four-character-code code-points used in this file-format family, as well as some other code-points related to MPEG-4 systems. The database is publicly viewable and registration is free .
 ISO/IEC 14496-12, ISO Base Media File Format; technically identical to ISO/IEC 15444-12
 ISO/IEC 21000-9, MPEG-21 File Format
 The MP4 Registration Authority, http://www.mp4ra.org/
 ISO/IEC 21000-2 Digital Item Declaration
 The Internet Assigned Numbers Authority http://www.iana.org/