Article

Understanding the HTTP Live Streaming Architecture

Understand how the major components of HLS work together to deliver streaming media.

Overview

Conceptually, HTTP Live Streaming consists of three parts: the server component, the distribution component, and the client software.

In a typical configuration, a hardware encoder takes audio-video input, encodes it as HEVC video and AC-3 audio, and outputs a fragmented MPEG-4 file or an MPEG-2 transport stream. A software stream segmenter then breaks the stream into a serious of short media files, which are placed on a web server. The segmenter also creates and maintains an index file containing a list of the media files. The URL of the index file is published on the web server. Client software reads the index, then requests the listed media files in order and displays them without any pauses or gaps between segments.

Flow diagram showing the four parts of an HTTP Live Stream. The media recording is sent to a server, distributed over the web, and sent to the client app.

Server Component

The server component is responsible for taking input streams of media and encoding them digitally. It encapsulates them in a format suitable for delivery and prepares the encapsulated media for distribution.

For live events, the server requires a media encoder, which can be off-the-shelf hardware, and a way to break the encoded media into segments and save them as files, which can either be software such as the media stream segmenter provided by Apple or part of an integrated third-party solution.

Distribution Component

The distribution system is a web server or a web-caching system that delivers the media files and index files to the client over HTTP. No custom server modules are required to deliver the content, and typically very little configuration is needed on the web server. To actually deploy HTTP Live Streaming, you need to create either an HTML page for browsers or a client app to act as a receiver. You also need the use of a web server and a way to encode live streams as fragmented MPEG-4 media files containing HEVC or H.264 video, and AAC or AC-3 audio.

Client Software

The client software is responsible for determining the appropriate media to request, downloading those resources, and then reassembling them so that the media can be presented to the user in a continuous stream. Client software is included in iOS 3.0 and later and on computers with Safari 4.0 or later installed.

The client software begins by fetching the index file, using a URL that identifies the stream. The index file, in turn, specifies the location of the available media files, decryption keys, and any alternate streams available. For the selected stream, the client downloads each available media file in sequence. Each file contains a consecutive segment of the stream. Once it has a sufficient amount of data downloaded, the client begins presenting the reassembled stream to the user.

The client is responsible for fetching any decryption keys, authenticating or presenting a user interface to allow authentication, and decrypting media files as needed.

This process continues until the client encounters the EXT-X-ENDLIST tag in the index file. If no EXT-X-ENDLIST tag is present, the index file is part of an ongoing broadcast. During ongoing broadcasts, the client loads a new version of the index file periodically. The client looks for new media files and encryption keys in the updated index and adds these URLs to its queue.

See Also

First Steps

Deploying a Basic HTTP Live Stream

Create a basic webpage to deliver an HTTP Live Stream.