HLS Workflow

In a typical HLS workflow, a video encoder solution that supports HLS receives a live video feed or distribution-ready media file. The encoder creates multiple versions (known as variants) of the audio/video at different bit rates, resolutions, and quality levels. The encoder then segments the variants into a series of small files, called media segments. At the same time, the encoder creates a media playlist file for each variant containing a list of URLs pointing to the variant’s media segments. The encoder also creates a master playlist file, containing a list of the URLs to variant media playlists, and descriptive tags to control the playback behavior of the stream. While producing playlists and segments, the encoder or automated scripts upload the files to a web server or CDN.

You provide access to the content by embedding a link to the master playlist file in a web page, or by creating your own custom application that downloads the master playlist file.

Media Segments

An encoder creates media segments by dividing the event data into short MPEG-2 transport stream files (.ts). Typically, the files contain H.264 video or AAC audio with a duration of 5 to 10 seconds each. The encoder lets you set the encoding and duration of the media segments.

Media Playlist Files

The encoder creates the media playlists as text files saved in the M3U format (.m3u8). The media playlists contain URLs to the media segments and other information needed for playback. The playlist type—live, event, or video on demand (VOD)—determines how the stream can be navigated.

Live playlists let viewers perform fast forward and reverse playback within a limited time range. The range advances along the program until the end of the live presentation. Event playlists let a viewer rewind to the beginning of the stream even as it continues to stream live. VOD playlists represent a previously completed program that can be fully navigated from beginning to end.

Live and Event Media Playlists

Both the live and event types of programs require updates to the media playlists as newly created media segments become available on the server. The encoder adds new media segment references to the end of the playlist and uploads the updated playlist to the server.

In a live playlist, as shown in Listing 1, references to older media segments can be removed from the media playlist and discarded, providing a sliding window into a continuous stream.

Listing 1  A simple live media playlist

#EXTM3U

#EXT-X-VERSION:6

#EXT-X-TARGETDURATION:10

#EXT-X-MEDIA-SEQUENCE:26

#EXTINF:9.901,

http://media.example.com/wifi/segment26.ts

#EXTINF:9.901,

http://media.example.com/wifi/segment27.ts

#EXTINF:9.501,

http://media.example.com/wifi/segment28.ts

An event playlist, shown in Listing 2, has the same format as the live media playlist above, with the exception of an extra line: #EXT-X-PLAYLIST-TYPE:EVENT. That designation alerts media player that this playlist will behave differently than a live media playlist. Event playlists maintain the references to older media while gaining new references. This process results in an expanding media playlist. This type of playlist allows the players to navigate freely (backward and forward) from the beginning of the program. Because references to all media segments remain in the playlist when the live event ends, event playlists are easily converted to VOD playlists.

Listing 2  A simple event media playlist

#EXTM3U

#EXT-X-VERSION:6

#EXT-X-TARGETDURATION:10

#EXT-X-MEDIA-SEQUENCE:0

#EXT-X-PLAYLIST-TYPE:EVENT

#EXTINF:9.9001,

http://media.example.com/wifi/segment0.ts

#EXTINF:9.9001,

http://media.example.com/wifi/segment1.ts

#EXTINF:9.9001,

http://media.example.com/wifi/segment2.ts

VOD Media Playlists

A VOD playlist, shown in Listing 3, contains references to all available media segments for the complete presentation, beginning to end. This kind of playlist allows the player to navigate the entire program. A VOD #EXT-X-ENDLIST tag marks the end of downloadable media segments.

Listing 3  A simple VOD media playlist

#EXTM3U

#EXT-X-VERSION:6

#EXT-X-TARGETDURATION:10

#EXT-X-MEDIA-SEQUENCE:0

#EXT-X-PLAYLIST-TYPE:VOD

#EXTINF:9.9001,

http://media.example.com/wifi/segment0.ts

#EXTINF:9.9001,

http://media.example.com/wifi/segment1.ts

#EXTINF:9.9001,

http://media.example.com/wifi/segment2.ts

#EXT-X-ENDLIST

Master Playlists

The master playlist provides an address for each individual media playlist in the stream. Figure 1 shows this relationship. The master playlist also provides important details such as bandwidth, resolution, and codec. The player uses that information to decide the most appropriate variant for the device and the currently measured, available bandwidth.

The sample master playlist in Listing 4 shows four variants. The order of the media playlists in the master playlists does not matter, except when you start the stream. The player begins downloading the first variant it can play. If conditions warrant, the player switches to another media playlist midstream.

Listing 4  A master playlist file with four available variants

#EXTM3U

#EXT-X-VERSION:6

#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=2855600,CODECS="avc1.4d001f,mp4a.40.2",RESOLUTION=960x540

live/medium.m3u8

#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=5605600,CODECS="avc1.640028,mp4a.40.2",RESOLUTION=1280x720

live/high.m3u8

#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1755600,CODECS="avc1.42001f,mp4a.40.2",RESOLUTION=640x360

live/low.m3u8

#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=545600,CODECS="avc1.42001e,mp4a.40.2",RESOLUTION=416x234

live/cellular.m3u8

A player downloads a master playlist only once. However, the number of media playlist downloads vary with the playlist type. For live and event broadcasts, the player downloads the media playlist files after each segment duration because the the playlist may update with new segments or lose older segments as the stream progresses. For VOD, players download media playlists only once.

Publishing Your HLS Streams

You provide access to your streams by adding a link to the master playlist file in a web page, or by creating your own custom application built with the AV Foundation or Media Player frameworks. See AVFoundation Programming Guide or Media Player Framework Reference.

Key Features

The HLS technology lets you stream your content through HTTP and enables automatic switching between streams as network conditions change. Those attributes alone make HLS a great solution for media distribution. Additionally, the technology includes features for usability, availability, advertising, and security. You can build the quality with the following extensible solutions.

Requirements for iOS Apps

The following requirements apply to iOS apps submitted for distribution in the App Store for use on Apple products. Noncompliant apps may be rejected or removed, at the discretion of Apple.

• If your app delivers video over cellular networks, and the video exceeds either 10 minutes or 5 MB of data in a five-minute period, you must use HLS. You can use progressive download for smaller clips.

• If your app uses HLS over cellular networks, you must provide at least one stream at 192 Kbps or lower bandwidth. The low-bandwidth stream may be audio-only or audio with a still image.

Sample Streams

A series of HTTP streams are available for testing on Apple's developer site. These examples show proper formatting of HTML to embed streams, .m3u8 playlists to index the streams, and .ts media segment files. See the HTTP Live Streaming page.

Apple provides simplified command-line tools, such as the Media File Segmenter and Media Stream Segmenter, to divide presentations and produce playlist files. Sign in with your developer ID and visit the HTTP Live Streaming page, under "Downloads."

Use the Apple-provided media stream validator found in these downloaded tools prior to serving your streams, to ensure that they are fully compliant with HLS.

Getting Started with HLS

To get started with your own HLS streams, the HTTP Live Streaming Content Guide provides a simple walkthrough of basic stream creation. The guide also explains complex HLS concepts, how to implement advanced features, and requirements for streaming to iOS devices.

Read the HTTP Live Streaming specification for current and definitive technical information.

HLS Feature Release History