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Softvelum Low Delay Protocol: low latency, high performance

Latency matters

Low latency has always been a hot topic for live streaming industry. Many use cases require sub-second delay between image capture and its display.

  • Chatting: talking to someone means immediate media delivery.
  • Gaming viewers expect to see all events real-time.
  • Betting and bidding have big money at stake so you don’t want to see anything behind the time.
  • Security and surveillance demand real-time picture to do an immediate response.
  • Second screen means you don’t want to see your favorite event dozens of seconds behind TV or other devices.

With the decline of Flash and RTMP in 2020 as the main way to view live streams, the industry has been looking for new ways to deliver the media close to real-time.

Softvelum introduces SLDP – a last-mile delivery protocol for end-user devices to cover all that.

What is SLDP

SLDP is a streaming protocol based on WebSockets. Basic features are:

SLDP clients support

SLDP is supported on various platforms with our playback solutions.

SLDP can be played in HTML5 browsers with MSE and MMS support using the SLDP HTML5 Player, and you can also add playback to your own players using the JavaScript SDK. Mobile support is available with Larix Player for Android and iOS.


HTML5 SLDP player
Freeware web player with mosaic videowall capabilities and more

You can use free apps and also purchase SDKs for these platforms to create your own solutions.

SLDP server support

Nimble Streamer freeware media server has full support for SLDP. It takes any protocol as input and creates SLDP output streams. It produces SLDP streams with H.264/H.265/AV1/VP8/VP9 video and AAC/MP3/Opus audio, and any SLDP player can connect to it for playback.

Amazon CloudFront has full support for WebSockets which means SLDP can be delivered via CloudFront network seamlessly. Take a look at CloudFront setup for SLDP delivery to learn more about proper settings.

Synchronized playback

SLDP technology allows performing synchronized simultaneous playback in multiple player instances, browsers and devices. This is important for cases like these:

  • One big screen with audio on individual devices.
  • A second screen application.
  • Multiple screens with single source of sound.
  • Surveillance cameras shown on the same page.

This capability is supported in Nimble Streamer server, HTML5 SLDP player, Larix Player for Android and iOS.
Synchronized playback setup article explains this feature in details.

You can try HTML5 demo page with two HTML5 SLDP players running in sync.
Demo video below shows players in HTML5 browser, Android and iOS devices, all playing the same live stream in sync with each other:

How does SLDP compare to others?

SLDP is good for low latency but how about other streaming protocols?


Chunks-based protocols were created with easy delivery in mind hence the chunks paradigm. This means that the client needs to download at least first chunk to start the playback. The default HLS chunk size recommended by Apple is 10 seconds but you can reduce it to 2-3 seconds. However, the delay cannot be reduced to sub-second values. The same applies to MPEG-DASH as it has the same nature.


Low-Latency HLS can be as low as 4 seconds latency if you want a reliable delivery of segments. We describe it here as “The minimum recommended part duration of 1000ms” which means 4 seconds end-to-end. Any lower part duration may bring your latency to 2 seconds, but that’s at your own risk. SLDP latency may go as low as 1-1.5 seconds in comparison.


WebRTC latency is lower than SLDP, however there are content-related limitations:

  • You can use any video and audio codec with SLDP, starting with H.264/AAC classic and also go with any other combination like VP9/MP3, HEVC/Opus etc. WebRTC is limited to H.264/Opus – this also requires additional transcoding if your original content is different.
  • You can use various codecs within the SLDP as part of the ABR stream too.
  • SLDP supports b-frames, while WebRTC doesn’t.

So with the trade-off to one second of latency (500ms for WebRTC to 1000-1500ms in SLDP), more options appear from the content perspective.


RTMP is an excellent real-time protocol greatly fit for low latency. However, it’s being declined by majors as it requires Flash for playback in the browsers.


Please also take a look at frequently asked questions to see advantages and best techniques of SLDP usage.

Check some snapshots of Softvelum usage to see how you can use SLDP with other products.

Try now

Now try it in your own use cases.

Contact us if you have any questions.