HTTP Live Streaming (also known as HLS) is an HTTP-based adaptive bitrate streaming communications protocol developed by Apple Inc. and released in 2009. Support for the protocol is widespread in media players, web browsers, mobile devices, and streaming media servers. As of 2022, an annual video industry survey has consistently found it to be the most popular streaming format. HLS resembles MPEG-DASH in that it works by breaking the overall stream into a sequence of small HTTP-based file downloads, each downloading one short chunk of an overall potentially unbounded transport stream. A list of available streams, encoded at different bit rates, is sent to the client using an extended M3U playlist. Based on standard HTTP transactions, HTTP Live Streaming can traverse any firewall or proxy server that lets through standard HTTP traffic, unlike UDP-based protocols such as RTP. This also allows content to be offered from conventional HTTP servers and delivered over widely available HTTP-based content delivery networks. The standard also includes a standard encryption mechanism and secure-key distribution using HTTPS, which together provide a simple DRM system. Later versions of the protocol also provide for trick-mode fast-forward and rewind and for integration of subtitles. Apple has documented HTTP Live Streaming as an Internet Draft (Individual Submission), the first stage in the process of publishing it as a Request for Comments (RFC). As of December 2015, the authors of that document have requested the RFC Independent Stream Editor (ISE) to publish the document as an informational (non-standard) RFC outside of the IETF consensus process. In August 2017, RFC 8216 was published to describe version 7 of the protocol.
Architecture HTTP Live Streaming uses a conventional web server, that implements support for HTTP Live Streaming (HLS), to distribute audiovisual content and requires specific software, such as OBS to fit the content into a proper format (codec) for transmission in real time over a network. The service architecture comprises:
Server Codify and encapsulate the input video flow in a proper format for the delivery. Then it is prepared for distribution by segmenting it into different files. In the process of intake, the video is encoded and segmented to generate video fragments and index file. Encoder: codify video files in H.264 format and audio in AAC, MP3, AC-3 or EAC-3. This is encapsulated by MPEG-2 Transport Stream (MPEG-TS) or MPEG-4 Part 14 (MP4) to carry it. Segmenter: divides the stream into fragments of equal length. It also creates an index file that contains references of the fragmented files, saved as .m3u8. Distributor Formed by a standard web server, accepts requests from clients and delivers all the resources (.m3u8 playlist file and .ts segment files) needed for streaming. Client Request and download all the files and resources, assembling them so that they can be presented to the user as a continuous flow video. The client software downloads first the index file through a URL and then the several media files available. The playback software assembles the sequence to allow continued display to the user.
Features HTTP Live Streaming provides mechanisms for players to adapt to unreliable network conditions without causing user-visible playback stalling. For example, on an unreliable wireless network, HLS allows the player to use a lower quality video, thus reducing bandwidth usage. HLS videos can be made highly available by providing multiple servers for the same video, allowing the player to swap seamlessly if one of the servers fails.
Adaptability To enable a player to adapt to the bandwidth of the network, the original video is encoded in several distinct quality levels. The server serves an index, called a master playlist, of these encodings, called variant streams. The player can then choose between the variant streams during playback, changing back and forth seamlessly as network conditions change.
Using fragmented MP4 At WWDC 2016 Apple announced the inclusion of byte-range addressing for fragmented MP4 files, or fMP4, allowing content to be played via HLS without the need to multiplex it into MPEG-2 Transport Stream. The industry considered this as a step towards compatibility between HLS and MPEG-DASH.
Low Latency HLS Two unrelated HLS extensions with a Low Latency name and corresponding acronym exist:
Apple Low Latency HLS (ALHLS) which was announced by Apple at WWDC2019 Community LHLS (LHLS) which predated Apple's publication and is allegedly simpler The remainder of this section describes Apple's ALHLS. It reduces the glass-to-glass delay when streaming via HLS by reducing the time to start live stream playbacks and maintain that time during a live-streaming event. It works by adding partial media segment files into the mix, much like MPEG-CMAF's fMP4. Unlike CMAF, ALHLS also supports partial MPEG-2 TS transport files. A partial media segment is a standard segment (e.g. 6 seconds) split into equal segments of less than a second (e.g. 200 milliseconds). The standard first segment is replaced by the series of partial segments. Subsequent segments are of the standard size. HTTP/2 is required to push the segments along with the playlist, reducing the overhead of establishing repeated HTTP/TCP connections. Other features include:
Playlist Delta Updates: only sending what changed between playlists, which typically fit in a single MTU making it more efficient to load the playlists which, with large DVR windows, can be quite large. Blocking of playlist reload: when requesting live media playlists, wait until the first segment is also ready, and return both at the same time (saving additional HTTP/TCP requests) Rendition Reports: add metadata to other media renditions to make switching between ABR faster New tags added: EXT-X-SERVER-CONTROL / EXT-X-PART / EXT-X-SKIP / EXT-X-RENDITION-REPORT URL QUERY_STRING ?_HLS callbacks added Apple also added new tools: tsrecompressor produces and encodes a continuous low latency stream of audio and video. The mediastreamsegmenter tool is now available in a low-latency version. It is an HLS segmenter that takes in a UDP/MPEG-TS stream from tsrecompressor and generates a media playlist, including the new tags above. Support for low-latency HLS is available in tvOS 13 beta, and iOS & iPadOS 14. On April 30, 2020, Apple added the low latency specifications to the second edition of the main HLS specification.
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