fMP4

Fragmented MP4 (fMP4) is a variation of the ISO Base Media File Format (ISOBMFF) in which media data is divided into small segments to enable streaming. Unlike a single, monolithic MP4 file, fMP4 is delivered as an initialization segment followed by multiple media fragments. Each fragment typically contains a Movie Fragment box (moof) and a Media Data box (mdat), while the initialization segment provides the file type box (ftyp) and the Movie Box (moov) that describe track metadata.

Structure and sequencing: The initialization segment contains ftyp and moov. Each subsequent fragment contains moof and mdat. The moof carries track fragment information (traf) and timing data (tfhd, tfdt, trun) that enables efficient random access, accurate seeking, and synchronization across multiple tracks. This design supports sending only the necessary fragments as playback progresses.

Usage and context: fMP4 is standard in streaming workflows such as MPEG-DASH and is commonly used with CMAF (Common Media Application Format) to provide a consistent container for DASH and modern HLS deployments. The fragmentation allows low-latency delivery, adaptive bitrate switching, and easier server-side caching and range requests.

Tools and compatibility: Popular encoding and packaging tools generate fMP4 segments, including FFmpeg, Bento4, and GPAC (MP4Box). Many modern browsers with Media Source Extensions (MSE) can assemble fMP4 fragments for playback, while older players may require a non-fragmented MP4 approach. fMP4 is a widely supported choice for streaming today.