Fragmented MP4 (fMP4) is the streaming-friendly variant of the MP4 container. A regular MP4 stores all its index metadata at the start or end of the file, which means a player has to download a whole big chunk before it can start playing. Fragmented MP4 instead splits the video into many small, independently-playable fragments — each with its own little index — so a player can start playing as soon as the first fragment arrives, and seek to any fragment without re-downloading the file.
This is the foundation of modern HTTP-based streaming. Both HLS (Apple's adaptive streaming protocol) and DASH (the MPEG standard) use fMP4 as their segment format. The same fMP4 segments can be served to both protocols simultaneously — that's what CMAF (Common Media Application Format) standardises, letting a single set of files reach every player from iPhone Safari to Roku to Chromecast.
For a streaming product, the practical implications are big. Fragments typically run 2–6 seconds; shorter fragments enable lower-latency live (LL-HLS pushes to ~1 second), longer ones improve compression efficiency and reduce playlist overhead. Every fragment must start with a keyframe (idr) so it's independently decodable, which couples segment length to GOP length. The fMP4 format also supports the byte-range-fetch tricks players use for adaptive bitrate — downloading just the next 2 seconds at whichever quality matches the current network speed. In 2026, fMP4 is the universal delivery container for streaming video.
