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. , 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, 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:

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: 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: 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.

Dynamic ad insertion

Dynamic ad insertion is supported in HLS using splice information based on SCTE-35 specification. The SCTE-35 splice message is inserted into the media playlist file using the EXT-X-DATERANGE tag. Each SCTE-35 splice_info_section is represented by an EXT-X-DATERANGE tag with a SCTE35-CMD attribute. A SCTE-35 splice out/in pair signaled by the splice_insert commands is represented by one or more EXT-X-DATERANGE tags carrying the same ID attribute. The SCTE-35 splice out command should have the SCTE35-OUT attribute and the splice in command should have the SCTE35-IN attribute. Between the two EXT-X-DATERANGE tags that contain the SCTE35-OUT and SCTE35-IN attributes respectively, there may be a sequence of media segment URIs. These media segments normally represent ad programs that can be replaced by the local or customized ad. The ad replacement does not require the replacement of the media files, only the URIs in the playlist need to be changed to point to different ad programs. The ad replacement can be done on the origin server or on the client's media-playing device.

Server implementations

Notable server implementations supporting HTTP Live Streaming include:

Usage

Supported players and servers

HTTP Live Streaming is natively supported in the following operating systems: Windows 10 used to have native support for HTTP Live Streaming in EdgeHTML, a proprietary browser engine that was used in Microsoft Edge (now referred to as Edge Legacy) before the transition to the Chromium-based Blink browser engine. Edge Legacy was included in Windows 10 up till version 2004. It was replaced by Edge Chromium in version 20H2. Along with Windows 11, Microsoft released an updated Media Player that supports HLS natively.

Clients

Servers

Live Encoders

VOD encoders