HTTP Live Streaming (also known asHLS) is anHTTP-basedadaptive bitrate streaming communications protocol developed byApple 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^[update], an annual video industry survey has consistently found it to be the most popular streaming format.^[2]

HLS resemblesMPEG-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 anextended M3U playlist.^[3]

Based on standard HTTP transactions, HTTP Live Streaming can traverse anyfirewall orproxy server that lets through standard HTTP traffic, unlike UDP-based protocols such asRTP. This also allows content to be offered from conventional HTTP servers and delivered over widely available HTTP-basedcontent delivery networks.^[4][5][6] The standard also includes a standard encryption mechanism^[7] and secure-key distribution usingHTTPS, which together provide a simpleDRM 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 anInternet Draft (Individual Submission), the first stage in the process of publishing it as aRequest 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 theIETF consensus process.^[8]In August 2017,RFC 8216 was published to describe version 7 of the protocol.^[9]

HTTP Live Streaming uses a conventionalweb server, that implements support for HTTP Live Streaming (HLS), to distribute audiovisual content and requires specific software, such asOBS to fit the content into a proper format (codec) for transmission in real time over anetwork.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 inH.264 format and audio inAAC,MP3,AC-3 orEAC-3.^[10] This is encapsulated byMPEG-2 Transport Stream (MPEG-TS) orMPEG-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 forstreaming.

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 aURL and then the several media files available. The playback software assembles the sequence to allow continued display to the user.

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.

To enable a player to adapt to the bandwidth of the network, the original video is encoded in several distinctquality levels. The server serves an index, called amaster playlist, of theseencodings, calledvariant streams. The player can then choose between the variant streams during playback, changing back and forth seamlessly as network conditions change.

At WWDC 2016 Apple announced^[11] the inclusion of byte-range addressing for fragmentedMP4 files, or fMP4, allowing content to be played via HLS without the need to multiplex it intoMPEG-2 Transport Stream. The industry considered this as a step towards compatibility between HLS andMPEG-DASH.^[12][13]

Two unrelated HLS extensions with aLow Latency name and corresponding acronym exist:

• Apple Low Latency HLS (ALHLS) which was announced by Apple at WWDC2019^[14]
• Community LHLS (LHLS) which predated Apple's publication and is allegedly simpler^[15]

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.^[16] 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. Themediastreamsegmenter 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.^[17]On April 30, 2020, Apple added the low latency specifications to the second edition of the main HLS specification.^[18]

Dynamicad insertion is supported in HLS using splice information based onSCTE-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.

Notable server implementations supporting HTTP Live Streaming include:

• Adobe Media Server supports HLS for iOS devices (HLS) and Protected HTTP Live Streaming (PHLS).
• Akamai supports HLS for live and on-demand streams.
• AT&T supports HLS in all formats live or on-demand.
• Ant Media Server support HLS and Low Latency HLS^[19] for live and on-demand streams.
• Axis Communication IP cameras supports HLS viaCamStreamer App ACAP
• Instart supports HLS for on-demand streams.
• Amazon CloudFront supports HLS for on-demand streams.
• Bitmovin supports HLS for on-demand and live streaming.
• CDNetworks supports HLS for live and on-demand streams.
• Cisco Systems: supports full end-to-end delivery for Live/TSTV/VOD/HLS and Cloud DVR services.
• Cloudflare supports HLS for live and on-demand streams.
• EdgeCast Networks supports cross-device streaming using HLS.
• Fastly supports HLS for live and on-demand streams.^[20]
• Helix Universal Server from RealNetworks supports iPhone OS 3.0 and later for live and on-demand HTTP Live or On-Demand streaming of H.264 and AAC content to iPhone, iPad and iPod.
• IIS Media Services fromMicrosoft supports live and on-demandSmooth Streaming and HTTP Live Streaming.
• Level 3 supports HLS live and on-demand streams.
• Limelight Networks supports HLS for some accounts.^[21]
• Nginx with the nginx-rtmp-module supports HLS in live mode. Commercial version Nginx Plus, which includesngx_http_hls_module module, also supports HLS/HDS VOD.^[22]
• Nimble Streamer supports HLS in live and VOD mode, Apple Low Latency HLS spec is also supported.
• Node.js with the hls-server package supports hls encoding to live mode and local files conversion.^[23]
• OvenMediaEngine is anopen source project that supports Low Latency HLS (LL-HLS) and HLS for live streaming.
• PeerTube supports HLS
• Storm Streaming Server supports HLS as backup mode for its Media Source Extensions player^[24]
• Tata CommunicationsCDN supports HLS for live and on-demand streams.
• TVersity supports HLS in conjunction with on-the-fly transcoding for playback of any video content on iOS devices.
• Ustream supports HLS delivery of live broadcasts. The ingested stream is re-transcoded if the original audio and video codec falls outside HLS requirements.
• VLC Media Player supports HLS for serving live and on-demand streams as of version 2.0.
• Wowza Streaming Engine fromWowza Media Systems supports HLS and encrypted HLS for live (with DVR), on-demand streaming and Apple Low Latency HLS spec.

• Google added HTTP Live Streaming support inAndroid 3.0 (Honeycomb).^[25]
• HP added HTTP Live Streaming support inwebOS 3.0.5.^[26]
• Microsoft added support for HTTP Live Streaming in EdgeHTML rendering engine in Windows 10 in 2015.^[27]
• Microsoft added support for HTTP Live Streaming in IIS Media Services 4.0.^[28]
• Yospace added HTTP Live Streaming support in Yospace HLS Player and SDK for flash version 1.0.^{[citation needed]}
• Sling Media added HTTP Live Streaming support to itsSlingboxes and its SlingPlayer apps.^[29]
• In 2014/15, theBBC introduced HLS-AAC streams for its live internet radio and on-demand audio services, and supports those streams with itsiPlayer Radio clients.^[30]
• Twitch uses HTTP Live Streaming (HLS) to transmit and scale the live streaming to many concurrent viewers, also supporting multiple variants (e.g., 1080p, 720p, etc.).^[31]

This list isincomplete; you can help byadding missing items.(March 2015)

HTTP Live Streaming is natively supported in the following operating systems:

• Windows 10 version 1507 to 2004 (Microsoft Edge Legacy) (no longer supported)
• Windows 11Media Player
• macOS 10.6+ (Safari andQuickTime)
• iOS 3.0+ (Safari)
• Android 4.1+^[32] (Google Chrome)

Windows 10 used to have native support for HTTP Live Streaming inEdgeHTML, a proprietary browser engine that was used in Microsoft Edge (now referred to as Edge Legacy) before the transition to the Chromium-basedBlink browser engine. Edge Legacy was included in Windows 10 up till version 2004. It was replaced by Edge Chromium in version 20H2. Along withWindows 11, Microsoft released an updatedMedia Player that supports HLS natively.

• Adaptive bitrate streaming
• Dynamic Adaptive Streaming over HTTP

• Hypertext Transfer Protocol
• Multimedia
• Network protocols
• QuickTime
• Streaming media systems

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