Live HLS protocol

HTTP Live Streaming（ abbreviation HLS） It's based on HTTP Streaming media network transmission protocol .​ It's apple QuickTime X and iPhone Part of a software system . Its working principle 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) file , Used to find available media streams .

HLS Just ask for basic HTTP message ,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 .

HLS protocol

HLS Provisions of the agreement

• The encapsulation format of video is TS.
• The video is encoded in H264, The audio encoding format is MP3、AAC perhaps AC-3.
• except TS The video file itself , It also defines the m3u8 file （ text file ）.

Client playback HLS The logic of video streaming is actually very simple , Download level 1 first Index file, It records the secondary index file （Alternate-A、Alternate-B、Alternate-C） The address of , Then the client downloads the secondary index file , The secondary index file records TS Download address of the file , So clients can download in order TS Video file and play continuously .

Class A index file

HLS Is to provide a m3u8 Address ,Apple Of Safari The browser can be opened directly m3u8 Address , for example ：

https://dco4urblvsasc.cloudfront.net/811/81095_ywfZjAuP/game/index.m3u8

 
Android It can't be opened directly , Need to use html5 Of video label , Then open this page in the browser , for example ：

```css
<!-- livestream.html -->
<video width="640" height="360"
        autoplay controls autobuffer 
        src="https://dco4urblvsasc.cloudfront.net/811/81095_ywfZjAuP/game/index.m3u8"
        type="application/vnd.apple.mpegurl">
</video> 

as well as m3u8 After the file is opened, the contents are as follows ：

#EXTM3U
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1064000
1000kbps.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=564000
500kbps.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=282000
250kbps.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=2128000
2000kbps.m3u8

bandwidth Specifies the bit rate of the video stream ,
PROGRAM-ID Useless without attention ,
every last #EXT-X-STREAM-INF The next line on the list is level two index Path to file , You can use relative path or absolute path . In the example, the relative path is used . This file records the secondary data of video streams with different bit rates index File path , The client can judge its current network bandwidth by itself , To decide which video stream to play . You can also smoothly switch to the video stream matching the bandwidth when the network bandwidth changes .

second level index file

#EXTM3U
#EXT-X-PLAYLIST-TYPE:VOD
#EXT-X-TARGETDURATION:10
#EXTINF:10,
2000kbps-00001.ts
#EXTINF:10,
2000kbps-00002.ts
#EXTINF:10,
2000kbps-00003.ts
#EXTINF:10,
... ...
#EXTINF:10,
2000kbps-00099.ts
#EXTINF:10,
2000kbps-00100.ts
#ZEN-TOTAL-DURATION:999.66667
#ZEN-AVERAGE-BANDWIDTH:2190954
#ZEN-MAXIMUM-BANDWIDTH:3536205
#EXT-X-ENDLIST

Level II documents are actually responsible for giving ts Download address of the file , Relative paths are also used here .

• EXTM3U
  Every M3U The first line of the document must be this tag, Provide marking function

• EXT-X-PLAYLIST-TYPE
  #EXT-X-PLAYLIST-TYPE:VOD The current video stream is an on-demand stream , That is to say, the whole of the video ts The file has been generated ,

• EXT-X-VERSION
  Used to mark the protocol version . Here is 3, So what's used here is HLS The third version of the agreement , This label can only have 0 or 1 individual , Not writing means using version 1

• EXT-X-TARGETDURATION
  #EXT-X-TARGETDURATION Specifies the maximum duration of the slice file in the current video stream , That is to say, these ts The length of slicing cannot be greater than #EXT-X-TARGETDURATION Value .

• EXT-X-MEDIA-SEQUENCE
  The start sequence number of the slice . Each slice has a unique serial number , Serial number between adjacent +1. This number will continue to grow , Ensure the continuity of flow .

• EXTINF
  ts Actual length of slicing .duration : Media duration

	#EXTINF <duration>,<title>

• EXT-X-ENDLIST
  #EXT-X-ENDLIST End of file symbol , Indicates the end of the video , No more media files are added to the playlist file. This flag also indicates that the current stream is a non live stream .

Play mode

on demand VOD The feature of is that you can get all the information at the current time point index Document and ts file , second level index All of them are recorded in the file ts File address . This client mode allows all content to be accessed . In the above example, it is an on-demand mode m3u8 Structure .

Live Pattern It's real-time generation M3u8 and ts file . Its index file has been in dynamic change , When playing, you need to constantly download secondary index file , To get the latest generated ts File play video . If a secondary index There is no... At the end of the file #EXT-X-ENDLIST sign , That means it's a Live Video streaming .

The client is playing VOD In fact, you only need to download level 1 video once index Documents and secondary index You can get all the files ts Download address of the file , Unless the client performs bit rate switching , Otherwise, there is no need to download any more index file , Just download in sequence ts File and play it . however Live Mode is slightly different , Because while playing , new ts Files are also being generated , So the client is actually downloading a secondary index file , Then download ts file , Download Level 2 index file （ At this time, this secondary index The file has been rewritten , The newly generated ts Download address of the file ）, Download new ts file , Play it over and over again .

HLS Main application scenarios

Cross platform ：PC The main live broadcast scheme is RTMP, There are also some libraries that can play HLS, for example jwplayer, be based on osmf Of hls There are also a lot of plug-ins . So in fact, if you choose a protocol, you can cross PC/Android/IOS, That's it HLS.

IOS Demanding stability requirements on ：IOS Of course, the most stable is HLS, The stability is not worse than RTMP stay PC-flash Performance on .

Amicable CDN Distribution method ： at present CDN about RTMP It's also the basic agreement , however HLS The basis of distribution is HTTP, therefore CDN Access and distribution will be better than RTMP More perfect . It can be used in various fields CDN Switch between ,RTMP Can also , It's just that docking tests may be required .

Simple ：HLS As a streaming media protocol, it is very simple ,apple The support is also very perfect .Android Yes HLS Our support will become more and more perfect . as for DASH/HDS, There seems to be no special reason , It's like linux Has become popular and open , Other systems are difficult to be widely used .

HLS And RTMP

RTMP finger Adobe Of RTMP(Realtime Message Protocol), It is widely used in low delay live broadcast , It is also the actual standard protocol for the connection between encoder and server , stay PC（Flash） It has the best viewing experience and the best stability .

HLS Mainly to solve RTMP There are some problems with the agreement . such as RTMP The protocol does not use standard HTTP The interface transmits data , So in some special network environment, it may be blocked by firewall . however HLS Because of the HTTP Protocol transfer data

RTMP There is a state of agreement , It's hard to scale the video server smoothly , Because it is necessary to maintain the status for each client playing the video stream . and HLS Based on stateless protocol （HTTP）, The client just downloads the common data stored in the server in order TS file , Being responsible and balanced is like ordinary HTTP File server load balancing is as simple as .

Besides ,HLS The protocol itself realizes rate adaptation , Different bandwidth devices can automatically switch to the most suitable video playback rate .

but HLS There are also some problems , Such as HLS The video delay of the protocol is large , Is basically 10 More than seconds , and RTMP The delay of the agreement can be up to 3、4 About seconds .

TS Protocol analysis part I （PAT）
TS Protocol analysis part II （PMT）
TS Protocol analysis part III （PES）
TS Protocol analysis part IV （adaptation field）