What is the m3u8 file format?

Introduction to m3u8

M3U8 It is the Unicode version M3U, encoded in UTF-8. Both "M3U" and "M3U8" files are the basis of the HTTP Live Streaming (HLS) protocol format used by Apple, which can be played on devices such as iPhones and Macbooks.

The above text definition comes fromWikipediaAs you can see, the m3u8 file is actually part of the HTTP Live Streaming (abbreviated as HLS) protocol, and HLS is an HTTP-based streaming media network transmission protocol proposed by Apple.

HLS works by breaking the entire stream into small HTTP-based files to download, downloading only a few at a time. While the media stream is playing, the client can choose to download the same resource from many different alternative sources at different rates, allowing the streaming session to adapt to different data rates. When starting a streaming session, the client downloads an extended M3U (m3u8) playlist file containing metadata for finding available media streams.
HLS only requests basic HTTP messages. Unlike the Real-time Transport Protocol (RTP), HLS can pass through any firewall or firewall that allows HTTP data to pass.actingserverIt is also easy to use content delivery networks to transmit media streams.

In short, HLS is a new generation of streaming media transmission protocol. Its basic implementation principle is to segment a large media file, record the resource path of the segmented file in the m3u8 file (i.e. playlist), and provide some additional descriptions (such as the multi-bandwidth information of the resource...) to the client. The client can obtain the corresponding media resource based on the m3u8 file and play it.

Therefore, the client obtains the HLS stream file mainly by parsing the m3u8 file.

So, let's briefly introduce the m3u8 file.

M3U8 File Introduction

The m3u8 file is actually a playlist, which may be a media playlist or a master playlist. Regardless of the type of playlist, the text inside it is encoded in utf-8.

When the m3u8 file is used as a media playlist, its internal information records a series of media segment resources. By playing the segment resources in sequence, the multimedia resources can be fully displayed. Its format is as follows:

#EXTM3U
#EXT-X-TARGETDURATION:10

#EXTINF:9.009,
http://media.example.com/first.ts
#EXTINF:9.009,
http://media.example.com/second.ts
#EXTINF:3.003,
http://media.example.com/third.ts
For on-demand playback, the client only needs to download the above segment resources in order and play them one by one. For live streaming, the client needs to re-request the m3u8 file at regular intervals to see if there is any new segment data that needs to be downloaded and played.

When m3u8 is used as the master playlist, it provides multiple stream list resources (Variant Stream) of the same media resource. Its format is as follows:

#EXTM3U
#EXT-X-STREAM-INF:BANDWIDTH=150000,RESOLUTION=416x234,CODECS="avc1.42e00a,mp4a.40.2"
http://example.com/low/index.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=240000,RESOLUTION=416x234,CODECS="avc1.42e00a,mp4a.40.2"
http://example.com/lo_mid/index.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=440000,RESOLUTION=416x234,CODECS="avc1.42e00a,mp4a.40.2"
http://example.com/hi_mid/index.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=640000,RESOLUTION=640x360,CODECS="avc1.42e00a,mp4a.40.2"
http://example.com/high/index.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=64000,CODECS="mp4a.40.5"
http://example.com/audio/index.m3u8
The backup stream resource specifies a variety of media playlists with different bit rates and formats, and the backup stream resource can also provide different versions of resource content at the same time, such as audio files in different languages, video files shot at different angles, etc. Customers can select resources with appropriate bit streams according to different network conditions, and it is best to select appropriate resource content according to user preferences.
The above is the general content of the m3u8 file. Next, we will explain the m3u8 content format.

m3u8 file format

The m3u8 file format mainly includes three aspects:

File playlist format definition: The content of the playlist (also known as the m3u8 file) must strictly meet the requirements of the specification definition. The following are some of the main conditions to be followed:
m3u8 files must be encoded in utf-8, cannot use the Byte Order Mark (BOM) byte order, and cannot contain utf-8 control characters (U+0000 ~ U_001F and U+007F ~ u+009F).

Each line of the m3u8 file is either a URI, a blank line, or starts with #.StringNo whitespace characters may appear, except in explicit declaration elements.

The string starting with # in the m3u8 file is either a comment or a tag. Tags start with #EXT and are case sensitive.

Attribute Lists: The values of certain tags are attribute lists. The attribute list after the tag uses commas as separators to separate multiple attribute/value pairs without spaces.
The syntax of the attribute/value pair is as follows:
AttributeName=AttributeValue
in:

The attribute AttributeName is an unquoted string consisting of [A..Z], [0..9] and -. Therefore, the attribute AttributeName can only use uppercase letters, not lowercase letters, and there cannot be a space between AttributeName and =. Similarly, there cannot be a space between = and AttributeValue.

The value of AttributeValue can only be of the following types:

Decimal integer: a decimal string without quotes consisting of [0..9], ranging from ~ (18446744073709551615), with a character length of 1 to 20.
Hexadecimal sequence: an unquoted string consisting of [0..9] and [A..F] prefixed with 0x or 0X. The maximum length of the sequence depends on its attribute name AttributeNames.
Signed-decimal-floating-point: An unquoted string consisting of [0..9], -, and .
String (quoted-string): A string enclosed in double quotes. 0xA, 0xD and double quotes " cannot appear in the string. The string is case sensitive.
Enumerated-string: A series of unquoted strings defined by AttributeName. The string cannot contain double quotes, commas, and whitespace characters.
decimal-resolution: two decimal integers separated by the character x. The first integer represents the horizontal width, and the second integer represents the vertical height (unit: pixel).
Tags: Tags are used to specify the global parameters of the m3u8 file or some information of the following slice file/media playlist.
There are five types of tags: Basic Tags, Media Segment Tags, Media Playlist Tags, Master Playlist Tags and Playlist Tags. The specific contents are as follows:

Basic Tags: Applicable to both Media Playlist and Master Playlist. The specific tags are as follows:

EXTM3U: Indicates that the file is an m3u8 file. Each M3U file must place this tag in the first line.
EXT-X-VERSION: Indicates the protocol version number of HLS. This tag is related to the compatibility of streaming media. This tag is global in scope and enables the entire m3u8 file. At most one definition of this tag can appear in each m3u8 file. If the m3u8 file does not contain this tag, the first version of the protocol is used by default.
Media Segment Tags: Each segment URI is preceded by a series of media segment tags to describe it. Some segment tags are only valid for the following segment resources; some segment tags are valid for all subsequent segments until another such tag is encountered. Media segment type tags cannot appear in the master playlist. The specific tags are as follows:

EXTINF: Indicates the duration (in seconds) of the media segment specified by the URL that follows. This tag must be specified before each URL media segment. The format of this tag is:

#EXTINF:,[