Introducing Low-Latency HLS

Crucial for Sports, Late-breaking news, Game Streaming, Award ceremonies, …

2-8 seconds is gold standard and matches cable/satellite TV

HLS targets 1-2 seconds delay

Regular HLS

• Simple and robust BUT comes with a cost (20-30s behind live)

• This comes due to client - server roundtrips and [too much caching] (https://developer.apple.com/videos/play/wwdc2019/502/?time=342)

Considerations

• HTTP is still king - but keeping it means keeping segment encode delay

• CDNs are essentially to scale

• Runway to react is much shorter

Changes

• Reduce Publishing Latency by allowing the server to publish small parts of the main segment before the main segment itself is ready

  • Leverage Partial Segments

  • Subset of regular segment - appear in parallel to them in playlist

  • Playlists update every partial segment

  • Publishing latency becomes the partial segment duration

  • Disappear quickly from playlist and are replaced/updated frequently

• Optimize Segment Discovery by allow asking for a particular playlist update in advance before it’s actually ready on the server

  • Features enabled by EXT-X-SERVER-CONTROL

    • Small number of server directives

    • Carried as query parameters on playlist URL (reserved in m3u8 URLs)

    • Sorted within URL to improve CDN hit ratio

• • Blocking Playlist Reload

    • EXT-X-SERVER-CONTROL with CAN-BLOCK-RELOAD control

    • Clients ask for playlist update in advance

    • Server holds reuqest until next (partial) segment appears

• Improve caching behavior by allowing to have a different URL for each update

• Eleminate Segment Round Trip by returning segments via push (HTTP/2)

  • A playlist GET request also pushes new segment which eleminates additional rount trip

• Reduce Playlist Transfer Overhead by transmitting only delta updates if requested

  • Realized via CAN-SKIP-UNTIL control

  • Clients ask for delta updates explicitly

  • Update skips the earlier part of playlist

• Switch Bitrate Tiers Quickly by transmitting additional information with the updates

  • Playlist carry up-to-date reports on peer playlists

  • Last media sequence number and last partial segment number

  • Allows client to load latest playlist when switching bit rates

How Do Get Started?

• Deliver HLS via HTTP/2 and support Push + Dependency and Weighting

• Each server must vend all bit rate tiers

• CDN must aggregate duplicate pending requests to origin

• Start implementing your origin - Spec for Low-Latency HLS is available now

• During Beta period use App Entitlement com.apple.developer.coremedia.hls.low-latency