Simple solution of m3u8 file format

m3u8 File format details

M3U8 yes Unicode Version of M3U, use UTF-8 code ."M3U" and "M3U8" The files are all used by apple HTTP Live Streaming（HLS） The basis of the protocol format , This protocol format can be used in iPhone and Macbook And other devices .

The above text definition comes from Wikipedia . You can see ,m3u8 The file is actually HTTP Live Streaming（ Abbreviation for HLS） Part of the agreement , and HLS It's based on HTTP Streaming media network transmission protocol .

HLS The working principle of is to divide the whole flow into small ones based on HTTP To download , Download only a few at a time . When the media stream is playing , Clients can choose to download the same resources at different rates from many different alternate sources , Allow streaming sessions to adapt to different data rates . When starting a streaming session , The client will download a extended M3U (m3u8) playlist file , Used to find available media streams .HLS Just ask for basic HTTP message , With real-time transmission protocol （RTP） Different ,HLS Can go through any permit HTTP Data through firewalls or proxy servers . It's also easy to use content distribution networks to stream media .

In short ,HLS It's a new generation of streaming media transport protocol , Its basic principle is to partition a large media file , Record the partition file resource path in m3u8 file （ namely playlist） Inside , With some additional descriptions （ For example, the multi bandwidth information of the resource ···） Used to provide to the client . The client uses the m3u8 File to get the corresponding media resources , Play it .

therefore , Client acquisition HLS Streaming files , Mainly right m3u8 File parsing operation .

that , Here is a brief introduction m3u8 file .

M3U8 File info

m3u8 The file is essentially a play list （playlist）, It could be a media play list （Media Playlist）, Or a master list （Master Playlist）. But no matter what kind of playlist , All the words inside are utf-8 code .

When m3u8 Files as media playlists （Meida Playlist） when , Its internal information records a series of media fragment resources , Play the clip resource in sequence , The multimedia resources can be displayed completely . The 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 VOD , The client only needs to download the above fragment resources in order , Play in sequence . And for live broadcast , The client needs Periodically re request The m3u8 file , See if there is new clip data to download and play .

When m3u8 As an anchor list （Master Playlist） when , It provides multiple stream list resources of the same media resource （Variant Stream）. The 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 standby stream resource specifies a variety of different bit rates , Media playlists in different formats , also , The standby stream resource can also provide different versions of resource content at the same time , For example, audio files in different languages , Video files taken from different angles, etc . Customers can select appropriate bitstream resources according to different network states , And it's best to choose the appropriate resource content according to user preferences .

Some precautions

• There are two kinds of requests m3u8 Playlist method ： One is to pass. m3u8 Of URI For the request , Then the document must be in .m3u8 or .m3u ending ; Two is through HTTP For the request , Then the request header Content-Type Must be set to application/vnd.apple.mpegurl perhaps audio/mpegurl.
• Empty lines and comment lines are ignored in parsing .
• Media play lists （Media Playlist） The total duration of stream resources is the sum of the duration of each slice resource .
• Bit rate of each slice （bit rate） Is the size of the slice divided by its corresponding duration （EXTINF A specified length of time ）.
• The same attribute in the attribute list of a tag AttributeName Only once .
• EXT-X-TARGETDURATION The specified duration must not be changed . Usually, the duration specified by this value is 10 second .
• For the designated EXT-X-I-FRAMES-ONLY And The first media segment （ Or the first one to follow EXT-X-DISCONTINUITY Fragments of ） Its resources do not immediately carry slices of the media initialization block , Labels should be added EXT-X-MAP Specify the media initialization block access .
• Use EXT-X-MAP The label contains labels EXT-X-I-FRAMES-ONLY Required compatible version number EXT-X-VERSION Be greater than or equal to 5; Use only EXT-X-MAP The required compatible version number must be greater than or equal to 6.
• By label EXT-X-MAP The declared media initialization block can be used AES-128 Method to encrypt , here , Act on EXT-X-MAP Labeled EXT-X-KEY The label must be set IV attribute .
• With attributes END-ON-NEXT=YES The label of EXT-X-DATERANGE Must carry CLASS attribute , But you can't carry DURATION and END-DATE attribute . The rest have the same CLASS The label of EXT-X-DATERANGE Overlapping date ranges cannot be specified .
• If the date range is not specified DURATION,END_DATE,END-ON-NEXT=YES Attribute , Then it will last （duration） Unknown , Even if it is set PLANNED-DURATION attribute .
• If the playlist is set EXT-X-DATERANGE label , Must be set at the same time EXT-X-PROGRAM-DATE-TIME label .
• If the playlist is set to have the same ID Two of the attribute values EXT-X-DATERANGE label , For the same attribute name , Between the two EXT-X-DATERANGE The corresponding values in must be consistent .
• If EXT-X-DATERANGE Also set up DURATION and END-DATE attribute , be END-DATE Attribute value must be equal to START-DATE Attribute value plus DURATION Property value .
• EXT-X-MEDIA-SEQUENCE The tag must appear before the first slice of the playlist .
• EXT-X-DISCONTINUITY-DEQUENCE The tag must appear before the first slice of the playlist .
• EXT-X-DISCONTINUITY-DEQUENCE The label must appear in any EXT-X-DISCONTINUITY Before the label .
• m3u8 File if not set EXT-X-PLAYLIST-TYPE label , Then the playlist can be changed at any time . such as , You can update or delete media clips in the playlist .
• Every EXT-X-I-FRAME-STREAM-INF The label must contain a BANDWIDTH and URI attribute .
• Every EXT-X-SESSION-DATA Labels must contain a VALUE or URI attribute , But not both .
• A playlist can contain multiple players with the same DATA-ID Attribute EXT-X-SESSION-DATA label . However, it cannot contain more than one carrying the same DATA-ID Same as LANGUAGE Attribute EXT-X-SESSION-DATA label .
• If set EXT-X-SESSION-KEY, So METHOD,KEYFORMAT and KEYFORMATVERSIONS The attribute value must be the same as any URI Of EXT-X-KEY The tag values are the same .
• If multiple backup streams or multilingual streams use the same encryption key and format , You should set EXT-X-SESSION-KEY label .
• The main playlist must not have multiple settings with the same METHOD,URI,IV,KEYFORMAT and KEYFORMATVERSIONS Attribute value EXT-X-SESSION-KEY label .