Deliver Low-Latency HLS live streams using Wowza Streaming Engine

Wowza Streaming Engine™ media server software version 4.7.8 and later can deliver Low-Latency HLS (LL-HLS) live streams. Low-Latency HLS streams conform to Apple’s preliminary protocol extension to the HLS spec.

About Low-Latency HLS streams


Wowza Streaming Engine generates Low-Latency HLS streams using the CMAF packetizer, cmafstreamingpacketizer. The packetizer creates chunks that are used as the video and audio partial segments needed for LL-HLS. The LL-HLS streams use the fMP4 container format and are delivered to players over HTTP/2.

Note: Using the CMAF packetizer includes some limitations. See About CMAF workflows in Wowza Streaming Engine for more information.

To deliver LL-HLS streams from Wowza Streaming Engine you have to manually enable low latency CMAF packetization for live streaming, in the application, by editing the application's configuration in XML.

Configure a live application to deliver LL-HLS streams


You can use the default live application that installs with Wowza Streaming Engine or create and use a new Live or Live HTTP Origin application. Use the Live application type for sending streams from Wowza Streaming Engine to clients or Wowza Streaming Engine edges. Use the Live HTTP Origin application type to connect to a CDN edge, such as the Fastly CDN, that can pull streams from the Wowza Streaming Engine origin application.

The following steps use the default live application to enable LL-HLS live streaming by editing the Application.xml and VHost.xml configuration files.

Notes:
  • In order to generate an LL-HLS streams you must first get a SSL/TLS certificate to secure your port and configure the port for HTTPS playback. LL-HLS playback does not work without an SSL/TLS secured port. For information on how to do this, refer to Get SSL/TLS certificates from the Wowza Streaming Engine StreamLock service.
  • If you're trying to use the CMAF packetizer to deliver LL-HLS and HLS or MPEG-DASH streams, you must use two separate live applications: one configured specifically for LL-HLS and another for CMAF-packetized HLS and MPEG-DASH.

Enable LL-HLS in Wowza Streaming Engine XML

The following steps enable transmuxing for CMAF packetization for LL-HLS delivery.

  1. Navigate to [install-dir]/conf/live and open the Application.xml file in a text editor.
  1. Add cmafstreamingpacketizer to the LiveStreamPacketizers property. You can add it to the prepopulated comma-separated list, or it can be the only packetizer specified. The XML looks like this:
<Streams>
    ...
    <LiveStreamPacketizers>cmafstreamingpacketizer</LiveStreamPacketizers>
    ...
</Streams>

Note: If you enable cupertinostreamingpacketizer and cmafstreamingpacketizer, Wowza Streaming Engine generates both MPEG-TS segments (using cupertinostreamingpacketizer) and fMP4 segments (using cmafstreamingpacketizer).

  1. Add the cmafLLEnableLowLatency property to the LiveStreamPacketizer  container element and set it to true.
<LiveStreamPacketizer>
    <Properties>
        <Property>
            <Name>cmafLLEnableLowLatency</Name>
            <Value>true</Value>
            <Type>Boolean</Type>
        </Property>
    </Properties>
</LiveStreamPacketizer>
  1. For the HTTPStreamers property, make sure HLS (cupertinostreaming) is specified. The XML looks like this:
<HTTPStreamers>cupertinostreaming</HTTPStreamers>
  1. Go to [install-dir]/conf/ and open the VHost.xml file in a text editor.
  2. Add the AllowHttp2 property to the <SSLConfig> container element in the <HostPort> you configured for SSL/TLS and set it to true.
<SSLConfig>
    ...
    <AllowHttp2>true</AllowHttp2>
    ...
</SSLConfig>
  1. Save your changes.

Your live application is configured to deliver LL-HLS streams. If desired, you can edit optional CMAF packetization properties. For information, see Configure CMAF live streaming packetization in Wowza Streaming Engine. You can also edit optional packetization properties to customize the LL-HLS partial segments and segments generated by the cmafstreamingpacketizer, or you can edit an optional property to customize the media playlist.

Configure optional LL-HLS properties


Configuring the following properties is optional as Wowza Streaming Engine will use the property's default value for any of the properties that you do not configure. You can configure the optional LL-HLS properties by editing the Application.xml file for your LL-HLS live stream application. For information about configuring custom (optional) properties, see Add custom properties.

Low latency CMAF packetization property reference

You can configure any of these optional LiveStreamPacketizer properties for the LL-HLS segments and video and audio partial segments (chunks) in the Application/LiveStreamPacketizer/Properties container element.

Note: If partial segments are too small, servers can become overloaded with frequent media playlist requests and cause playback to be less reliable.
Name Type Description
cmafLLChunkingScheme String Specifies the chunking scheme for low latency CMAF-packetized streams. Valid values are byFrame or byDuration. The default is byDuration.

If the value is byDuration, packetization is configured using the cmafLLChunkDurationTargetAudio and
cmafLLChunkDurationTargetVideo properties.

If the value is byFrame, packetization is configured using the cmafLLChunkFrameCountTargetAudio and 
cmafLLChunkFrameCountTargetVideo properties.
cmafLLChunkDurationTargetAudio Integer Specifies, in milliseconds, the target duration of each low latency CMAF audio chunk. The default is 1000 and is recommended. The duration cannot exceed the cmafSegmentDurationTarget value. This property is only available if cmafLLChunkingScheme is byDuration
cmafLLChunkDurationTargetVideo Integer Specifies, in milliseconds, the target duration of each low latency CMAF video chunk. The default is 1000 and is recommended. The duration cannot exceed the cmafSegmentDurationTarget value. This property is only available if cmafLLChunkingScheme is byDuration.
cmafLLChunkFrameCountTargetAudio Integer Specifies the target number of audio frames to include in each low latency CMAF audio chunk. The default is 47 and is recommended. This property is only enabled if cmafLLChunkingScheme is byFrame.
cmafLLChunkFrameCountTargetVideo Integer Specifies the target number of video frames to include in each low latency CMAF video chunk. The default is 30 and is recommended. This property is only enabled if cmafLLChunkingScheme is byFrame.
cmafSegmentDurationTarget Integer Specifies, in milliseconds, the duration of the fMP4 segments in the stream. The default is 6000 (6 seconds) and is recommended. This property is configurable from the CMAF Streaming Packetizer properties section in Wowza Streaming Engine Manager.

Advanced LL-HLS property reference

For advanced tuning, you can configure the following optional HTTPStreamer property in the Application/HTTPStreamer/Properties container element.

Name Type Description
cupertinoPartHoldBack
(4.8.0 and later)
Double Specifies, in floating-point seconds, the server-recommended minimum distance from the live edge at which clients should begin to play or seek in a LL-HLS stream (the PART-HOLD-BACK attribute in media playlists). If you do not configure this property, Wowza Streaming Engine uses the default value, which is recommended. The default value is calculated as three times the maximum value of PART-TARGET out of all of a stream's variant media playlists.
 
Note: PART-TARGET is an attribute of the EXT-X-PART-INF tag in media playlists and is based on the target values set with the cmafLLChunkDurationTargetVideo and cmafLLChunkDurationTargetAudio properties or the cmafLLChunkFrameCountTargetVideo and cmafLLChunkFrameCountTargetAudio properties.
Wowza Streaming Engine will adjust the PART-HOLD-BACK value to be equal to the maximum value of PART-TARGET (across all variant media playlists) if you set this property to less than a partial segment duration.

Transcoding considerations for LL-HLS


To bypass encoding streams with Transcoder, source streams should meet the following encoding recommendations. Otherwise, transcoding is recommended. Although a small amount of latency is introduced with transcoding, if your source stream does not meet the encoding recommendations, transcoding will ultimately improve reliability of LL-HLS streaming.

Encoding recommendations

  • CMAF-supported codecs (required)
     
    Video Audio
    • H.264, H.265
    • AAC, AAC-LC, HE-AAC (AAC+ or aacPlus), HE-AACv2 (enhanced AAC+, aacPlus v2)
    • Dolby Digital 5.1 Surround Sound (AC-3) and Dolby Digital Plus (Enhanced AC-3 or E-AC-3)

     
  • GOP size: 1 or 2 seconds
  • Closed GOPs
  • H.264 and H.265 byte streams contain metadata about the GOP structure
  • Constant frame rate video

A stream configured for transcoding and LL-HLS delivery cannot use Passthrough for the Video Codec of a stream rendition but not other renditions of the same stream. If you use Passthrough to pass a video source through to output without making any changes for any Video Codec in a stream's renditions, all of the video codecs must be set to Passthrough.

Stream name groups (NGRP) are not supported with LL-HLS. To group multiple stream renditions for LL-HLS, you need to create a SMIL file in accordance with CMAF HLS. See Set up adaptive bitrate CMAF streaming for how to do this.

Tune for latency


In addition to configuring the CMAF properties, there are steps you can take to tune streams for low latency, depending on the source and whether it needs to be transcoded.

For an RTSP source stream:

  • In [install-dir]/conf/[application-name]/Application.xml, in the <Streams> container, set <StreamType> to rtp-live-lowlatency.
<Streams>
	<StreamType>rtp-live-lowlatency</StreamType>
        ...
</Streams>

For an RTMP source stream:

  • In [install-dir]/conf/[application-name]/Application.xml, in the <Streams> container, set <StreamType> to live-lowlatency.
<Streams>
	<StreamType>live-lowlatency</StreamType>
        ...
</Streams>

For any transcoded stream:

  • In your transcoder template in [install-dir]/transcoder/templates, disable the <SortBuffer> in the <PostProcess> container element by setting <Enable> to false.
<PostProcess>
	<SortBuffer>
		<Enable>false</Enable>
		...
	</SortBuffer>
</PostProcess>

Scale LL-HLS with a CDN


Optionally, with your own CDN account, you can scale LL-HLS delivery by connecting a Wowza Streaming Engine Live HTTP Origin application to a CDN edge that can pull streams from the Wowza Streaming Engine origin application, such as the Fastly CDN.

To connect the Live HTTP Origin application to your CDN, provide the IP address or hostname of the Wowza Streaming Engine origin server in your CDN configuration. Also in your CDN configuration, enable connecting to the origin server over SSL/TLS using the port you configured for SSL/TLS in the Wowza Streaming Engine origin (default port 443).

Test stream playback


After your live application is configured to deliver an LL-HLS stream, complete the stream setup by connecting a source encoder or IP camera to Wowza Streaming Engine and publishing the live source stream to the server. When the camera or encoder is connected and the live stream is active, test LL-HLS playback using a supported test player.

You can test LL-HLS streams by entering the playback URL in the Safari browser on iOS 14, other native apps on devices running iOS 14, or any player that supports LL-HLS. If a player supports HLS but not LL-HLS, it will default to playing LL-HLS streams as standard HLS.

To test LL-HLS live streams when only cmafstreamingpacketizer is enabled, specify the stream playback URL using one of the following formats: 

Single bitrate

https://[address]/[application]/[application-instance]/[stream-name]/playlist.m3u8

Adaptive bitrate

https://[address]/[application]/[application-instance]/smil:[stream-name].smil/playlist.m3u8

If cmafstreamingpacketizer and cupertinostreamingpacketizer are enabled, specify the stream playback URL using one of the following formats:

Single bitrate

https://[address]/[application]/[application-instance]/[stream-name]/playlist_sfm4s.m3u8

Adaptive bitrate

https://[address]/[application]/[application-instance]/smil:[stream-name].smil/playlist_sfm4s.m3u8

Where:

  • [address] is the IP address or domain and port (default port 443)
  • [application] is the application name
  • [application-instance] is the name of the application instance (if omitted, defaults to _definst_)
  • [stream-name] is the stream name

So, for example, if only cmafstreamingpacketizer is enabled, the playlist URL for an adaptive bitrate LL-HLS stream that uses the address example.com, the default live application, and the default stream name myStream is:

https://example.com/live/smil:myStream.smil/playlist.m3u8

If cmafstreamingpacketizer and cupertinostreamingpacketizer are both enabled, the adaptive bitrate LL-HLS playlist URL for the same example is:

https://example.com/live/smil:myStream.smil/playlist_sfm4s.m3u8

and the playback URL for the Cupertino HLS stream is either:

https://example.com/live/smil:myStream.smil/playlist.m3u8

or

https://example.com/live/smil:myStream.smil/playlist_sfts.m3u8

where _sfts indicates that the media playlist contains .ts segments.

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