Reduce live stream latency

Mux Video live streaming is built with RTMP ingest and HLS delivery. HLS inherently introduces latency. To the broadcasting industry, this latency is called glass-to-glass latency. Standard glass-to-glass latency with HLS is greater than 20 seconds and typically about 25 to 30 seconds.

To clarify some terminology and industry jargon:

• Glass-to-glass latency: Also sometimes referred to as end-to-end latency. This latency is defined as the time lag between when a camera captures an action and when that action reaches a viewer’s device.
• Wall-clock time: Also might be referred to as "realtime". If you have a clock on the wall where you are capturing video content, this would be the time on that clock.

The nature of HLS delivery means that clients are not necessarily synchronized. Some clients might be 15 seconds behind wall-clock time and others might be 30 seconds behind.

Where does the latency come from?

You don't have to worry about these gritty details when using Mux for live streams, but to give you an idea of how a live stream works:

1. Captured by a camera
2. Processed by an encoder - If the computer running the encoder is running out of CPU this process can get behind and start lagging.
3. Send to an RTMP ingest server - This server is ingesting the video content in real-time. This part is called the "first mile", it's happening over the internet, often times on consumer or cellular network connections so things like TCP packet-loss and random network disconnects are always happening.
4. Ingest server decodes and encodes - Assuming all the content is traveling over the internet fast enough, the encoder on the other end needs to keep up and have enough CPU available to package up segments of video as they come in. The encoder has to ingest video, build up a buffer of content and then start decoding, processing and encoding for HLS delivery.
5. Manifest files and segments of video delivered - After all of that, files are created and delivered over HTTP through multiple CDNs to reach end users. Each file becomes available after the entire segment's worth of data is ready. This part also happens over the internet where the same risks around packet-loss and network congestion are factors. Network issues are especially a factor for the last mile of delivery to the end user.
6. Decoded and played on the client - When video makes it all the way to the client. The player has to decode and playback the video. Players do not play each segment on the screen as they receive it, they keep a buffer of playable video in-memory which also contributes to the glass-to-glass latency experienced by the end user.

When you consider each of the steps above, any point of that pipeline has the potential to slow down or get backed up. The more latency you can tolerate, the safer the system is and the lower probability you have for an unhappy viewer. If any single step gets backed up momentarily, the whole system has a chance to catch up before an interruption in playback. And, when everything is running smoothly, the player has extra time to spend downloading the higher quality version of your content.

Reconnect windows

When an end-user is streaming from their encoder to Mux, we need to know how to handle situations when the client disconnects unexpectedly.

There are situations when a client disconnects on purpose: for example hitting "Stop streaming" on OBS. Those are intentional disconnects, we're talking about times when the client just stops sending video. In order to handle this, live streams have a reconnect_window. After an unexpected disconnect, Mux will keep the live stream "active" for the given period of time and wait for the client to re-connect and start streaming again.

When the reconnect_window expires the live stream will transition back into the idle state. In HLS terminology, Mux will write the #EXT-X-ENDLIST tag to the HLS manifest. At this point, your player will consider the live stream to be over.

By default reconnect_window is 60 (seconds) - you can set this as high as 300 (5 minutes).

Lower latency Options

Mux live streams have options for "reduced latency" and "low latency". The reduced_latency option gets your latency down to a range of 12-20 seconds and the low_latency BETA further reduces the latency to a range of 4-7 seconds. But your viewers might see some variance to the glass-to-glass latency because the latency depends on many factors, including player configurations, your viewer's geographic location, and their internet connectivity speed.

When either latency option is enabled, Mux treats your live stream input over RTMP a little differently. These are two important considerations when enabling the lower latency options:

1. No reconnect windows - without reconnect windows, if your encoder disconnects for any reason, the live stream will transition to the idle state. A corresponding asset is created as the live stream segment recording and the HLS manifest indicates that the stream has ended (#EXT-X-ENDLIST tag). If the encoder later reconnects, your player has to properly reload to start streaming from your stream URL.

2. Potentially less stability - because of the tradeoffs mentioned above, both reduced_latency & low_latency BETA option can inherently be less stable than standard latency. Any delay can impact the stability of the live stream playback due to internet connectivity on the first-mile or last-mile or higher than average resource utilization on the encoder or the video player devices.

Input Requirements

You should only use either of the flags if you have control over the following:

• the encoder software
• the hardware the encoder software is running on
• the network the encoder software is connected to

Typically, home networks in cities and mobile connections are not stable enough to reliably use either of the latency options.

Create a live stream with the reduced_latency option

Check out our Live Stream API ReferenceAPI and find the reduced_latency flag and include "reduced_latency": true in the request to create a live stream.

Here is how to add the flag to a live stream by making this POST request:

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POST https://api.mux.com/video/v1/live-streams
{
    "reduced_latency": true,
    "playback_policy": ["public"],
    "new_asset_settings": {
        "playback_policy": ["public"]
    }
}  

You can read more about the reduced latency feature in this blog post about Reduced Latency.

Create a live stream with the low_latency BETA option

The same Create Live StreamAPI is also used to set the low_latency BETA flag.

Here is how to add the flag to a live stream by making this POST request:

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POST https://api.mux.com/video/v1/live-streams
{
    "low_latency": true,
    "playback_policy": ["public"],
    "new_asset_settings": {
        "playback_policy": ["public"]
    }
}  

Low-latency FAQs

Is low-latency HLS different from standard latency HLS?

Yes. The low_latency BETA mode uses Apple's new low-latency HLS (LL-HLS) spec that allows your viewers to play live streams with as low as 4 seconds glass-to-glass latency. In comparison, the other latency modes are based on the earlier version of the HLS spec which puts a cap on the lower limits of the glass-to-glass latency.

Mux has closely followed the new low-latency HLS spec and published about the development of the new spec a few times on our blog in June 2019 and again in January 2020.

Do video players support low-latency HLS?

Yes. Because low_latency BETA mode uses Apple's new LL-HLS spec, you need to upgrade your video player. Below is the list of the most commonly used video players and the minimum version.

My video player does not support the LL-HLS spec. Can it still play a low-latency live stream?

Maybe. Apple's LL-HLS specification is backward compatible. So your video player should fall back to playing standard HLS. Those viewers will have noticeably higher glass-to-glass latency. However, your video player does need support for demuxed audio & video tracks (each track requested separately) in MP4 format for being backward compatible. Most video players already support demuxed audio & video tracks in MP4 format.

My video player is running into issues when playing a low-latency live stream. Can I play the same live stream without the low-latency?

Yes. You can add the low_latency=false parameter to the video playback URL. Mux can revert back to delivering the same live stream using standard HLS by adding this low_latency=false parameter. However, your video player does need support for demuxed audio & video tracks (each track requested separately) in MP4 format for the low_latency=false parameter to work.

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https://stream.mux.com/{PLAYBACK_ID}.m3u8?low_latency=false  

If your playback_id is signed, then all query parameters, including low_latency BETA need to be added to the claims body. Take a look at the signed URLs guide for details.

How do you enable LL-HLS playback on VideoJS player?

You can enable low latency playback using Apple's LL-HLS spec by initializing videojs-http-streaming module with the experimentalLLHLS flag along with any other options.

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var player = videojs(video, {
  html5: {
    vhs: {
      experimentalLLHLS: true
    }
  }
});