HTTP Live Streaming

HTTP Live Streaming (also known as HLS) is an HTTP-based media streaming communications protocol implemented by Apple Inc. as part of its QuickTime, Safari, OS X, and iOS software. It resembles MPEG-DASH in that it works by breaking the overall stream into a sequence of small HTTP-based file downloads, each download loading one short chunk of an overall potentially unbounded transport stream. As the stream is played, the client may select from a number of different alternate streams containing the same material encoded at a variety of data rates, allowing the streaming session to adapt to the available data rate. At the start of the streaming session, HLS downloads an extended M3U playlist containing the metadata for the various sub-streams which are available.^[2]

HLS Live Stream Example http://fish.schou.me

Since its requests use only 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 as origin and delivered over widely available HTTP-based content delivery networks.^[3]

HLS also specifies a standard encryption mechanism^[4] using AES and a method of secure-key distribution using HTTPS with either a device-specific realm login or HTTP cookie 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. upLynk has also added the AES scrambling and base-64 encoding of the DRM content-key with a 128-bit device-specific key for registered commercial SWF applications together with a sequential initialization Vector for each chunk to its implementation of the standard.^[5]

Apple has documented HTTP Live Streaming as an Internet Draft (Individual Submission), the first stage in the process of submitting it to the IETF as an Informational Request for Comments. However, while Apple has submitted occasional minor updates to the draft, no additional steps appear to have been taken towards IETF standardization.^[6]

Architecture

HTTP Live Streaming uses a conventional web server to distribute audiovisual content and requires specific software to fit into the proper format transmission in real time. 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 coded and segmented to generate video fragments and index file.

• Encoder: codify video files in H.264 format and audio in MP3, HE-AAC or AC-3. This is encapsulated by MPEG-2 Transport Stream to carry it
• Segmenter: divides the MPEG-2 TS file into fragments of equal length, kept as .ts files. It also creates an index file that contains references of the fragmented files, saved as .m3u8

Distributor

Formed by standard web Server, accepts requests from clients and delivers all the resources 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 to provide a scalable and adaptable to network, allowing playback quality in wireless networks with high bandwidth and low quality playback on 3G networks, where the bandwidth is reduced. HTTP Live Streaming also provides protection against errors, generating alternative different flows video to use them if there are any errors in segment.

HLS streams can carry generic ID3 data as a separate pid in the transport stream (see ^[7]). ID3 metadata is specified by Apple in separate audio streams for the purposes of synchronisation with video. Timed ID3 metadata in the base streams can be used to carry generic timed metadata with the stream; for example Screen Systems have used ID3 information to reference the time that the frame on the client's system was encoded, allowing code external to the player to act in sync with the video. The number of players able to export the ID3 information is growing, examples being hls.js, flashls (flash plugin which works with a number of flash players), and JWPlayer (paid version).

Scalability

To make the system scalable and adaptable to the bandwidth of the network, the video flow is coded in different qualities. Thus, depending on the bandwidth and transfer network speed, the video will play at different qualities.

To implement this, the system must encode the video in different qualities and generate an index file that contains the locations of the different quality levels.

The client software internally manages the different qualities, making requests to the highest possible quality within the bandwidth of the network. Thus always play the video the highest possible quality, viewing lower quality on 3G networks and highest quality in Wi-Fi broadband.

One issue that can arise by relying on the client-side system is that a user may experience different bitrates throughout the duration of playback. This can be avoided by delaying the responsiveness of the player to changes in client-side bandwidth.

Availability

To keep a stream available HLS includes features to recover from outages. To achieve this, multiple flows are listed in the index file for the same quality level. If the client can't load a flow it tries the next, repeating until either a working flow is found or all flows fail.

This can be combined with scalability by listing multiple flows for each separate quality.

Using fragmented MP4

On WWDC2016 Apple announced^[8] the inclusion of byte-range addressing for fragmented MP4 files, or fMP4, allowing content to be played in 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.^[9][10]

Server implementations

• Adobe Media Server supports HLS for iOS devices (HLS) and Protected HTTP Live Streaming (PHLS).
• Akamai supports HLS for live and on-demand streams.
• Amazon CloudFront supports HLS for on-demand streams.
• bitmovin provides the cloud transcoding service bitcodin.com which 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.
• EdgeCast Networks supports cross-device streaming using HLS.
• Edgeware: Supports full end to end delivery for Live/TSTV/VOD/HLS.
• Exterity: AvediaStream o7500 Origin Server supports HLS (HTTP Live Streaming)
• Flussonic Media Server^[11] supports multibitrate HLS either for transcoding or streaming since 4.3.0 version^[12]
• 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 from Microsoft supports live and on-demand Smooth Streaming and HTTP Live Streaming.
• Keepixo Genova Live does both transcoding to H.264 & HEVC and packaging to HLS, MPEG-DASH, MS Smooth Streaming
• Level 3 supports HLS live and on-demand streams.
• Limelight Networks supports HLS for some accounts.^[13]
• Nginx with the nginx-rtmp-module supports HLS in live mode.
• Nimble Streamer supports HLS in live and VOD mode.
• Tata Communications CDN 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.
• Unified Origin supports HLS in live and VOD mode.
• Unreal Media Server supports low latency HLS as of version 9.5.
• 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 from Wowza Media Systems supports HLS and encrypted HLS for live (with DVR) and on-demand streaming.

Usage

• Adobe Systems demonstrated an update to its Adobe Flash Media Server product supporting HTTP Live Streaming at the NAB Show in April 2011
• Apple Inc. used this on September 1, 2010 to stream its iPod Keynote event live over the internet, and on October 20, 2010 to stream its 'Back to the Mac' Keynote event live over the internet.
• Google added HTTP Live Streaming support in Android Honeycomb and later.^[14]
• 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 initial release April 2010, latest release November 2012
• flashls is an open-source Adobe Flash/Adobe AIR plugin that provides HTTP Live Streaming support for Chromeless Flash/Air Player, Flowplayer, Video.js, mediaelement.js and Open Source Media Framework 2.0
• HP added HTTP Live Streaming support in webOS 3.0.5.^[15]
• Microsoft added support for HTTP Live Streaming in EdgeHTML rendering engine in Windows 10 in 2015^[16]
• Microsoft added support for HTTP Live Streaming in IIS Media Services 4.0.^[17]
• Yospace added HTTP Live Streaming support in Yospace HLS Player and SDK for flash version 1.0
• Sling Media added HTTP Live Streaming support to its Slingboxes and its SlingPlayer apps^[18]
• In 2014/15, the BBC introduced HLS-AAC streams for its live internet radio and on-demand audio services, and supports those streams with its iPlayer Radio clients.^[19]
• hls.js implements HLS in MSE compatible browsers; ability to read ID3 from video pid ^[20][21]
• hasplayer.js implements HLS in MSE compatible browsers^[22]

Supported players and servers

HTTP Live Streaming is natively supported in the following operating systems: Windows 10 (Microsoft Edge), OS X 10.6+ (Safari and QuickTime), iOS 3.0+ (Safari), and Android 4.1+^[23] (Google Chrome).

Clients

Servers

Live Encoders

VOD encoders

See also

• Adaptive bitrate streaming
• Dynamic Adaptive Streaming over HTTP

References