Apple's low latency HLS (alhls) aims to reduce live streaming delays, currently averaging 25 to 30+ seconds, with various technical innovations including partial segments and faster playlist requests. The document discusses the challenges and components involved in implementing alhls, such as smaller segment durations and dynamic origin APIs. It emphasizes the ongoing learning process in the industry as alhls is being tested and refined, with the potential to set a new standard for low-latency streaming.

1. What’s the deal with Apple’s Low Latency HLS (ALHLS)?
Zac Shenker
Director of Engineering, Video Delivery, Optimization & Experience
zac.shenker@cbsinteractive.com

2. Current Latency @ CBSi
• 6 Second Segments (2 Second GOPs)
– Some BUs testing 2 Second Segments
• Players Start 3 Segments Behind Live Edge
• Minimum Latency - 18 seconds
• Real-World Latency - 25 to 30+ seconds
• Proven at Scale to Millions of Concurrent Viewers

3. Other Sources of Latency
• Playlist Fetches
– Generally runs at half segment duration (3 seconds)
• Broadcast Distribution
– Satellite Latency
– Multiple Generations of Encode/Decode

4. Latency Tradeoffs
• Maximum Player Buffer Length
• Rendition / Quality Level
• Risk of Rebuffering
• Stream Startup Time
• Non-Simulcast or Interactive Content

5. HLS Low Latency before Apple
• Custom Tags in HLS Manifests
– Designed to be backwards compatible
• HTTP/1.1 Chunked Transfer Coding for Segments
• Bandwidth Estimation Modifications Needed
• Implementations from Periscope & Twitch
• HLS.JS RFC - https://github.com/video-dev/hlsjs-rfcs/pull/1
• Approaches for DASH CMAF too (not covering this today)

6. Disclaimer
• This is New to Everyone
• We are Learning and Experiment across the Industry
• Scaling Live is Hard
– In General (even without Low Latency)
– At Scale
– While Maintaining Great QoE
– Across an Increasing Number of Devices / Platforms

7. References for ALHLS
• WWDC Video
https://developer.apple.com/videos/play/wwdc2019/502/
• Preliminary Spec
https://developer.apple.com/documentation/http_live_streaming/protocol_extension_for_low-
latency_hls_preliminary_specification
• Blog Post - Phil Harrison (AWS)
https://www.linkedin.com/pulse/apples-implementation-low-latency-hls-explained-phil-harrison/
• Blog Post - Phil Cluff (Mux)
https://mux.com/blog/the-community-gave-us-low-latency-live-streaming-then-apple-took-it-away/

8. Considerations for ALHLS
• HTTP is Standard for Online Video Delivery
– Segments must become smaller (250ms)
• CDNs are essential for scale
– CDNs are good at caching, we need to use that
• Runway to react is shorter
– Switch renditions in 600ms instead of 10s

9. Origin API
• #EXT-X-SERVER-CONTROL to Advertise Server Support
− CAN-BLOCK-RELOAD=YES is Mandatory
• _HLS* Query Parameters Reserved
− Query Params should be Sorted
• Client Requests Different Responses/Behaviors from Origin
• CDN must Cache based on Query Params

10. 5 Components of ALHLS
• Partial Segments
• Reducing Latency of Segments
• Blocking Playlist Requests
• Playlist Delta Updates
• Faster Rendition Switches

11. 1 - Partial Segments
• Today
– 6 Second Segment Only Advertised when Fully Available
• Partial Segment every 250ms
– Full Segment Must be Available before Advertised
• New #EXT-X-PART tag
– #EXT-X-PART:DURATION=0.33334,URI="filePart271.4.ts",INDEPENDENT=YES

12. 1 - Partial Segments (Challenges)
• Parts Advertised when Fully Available
• Parts in Manifest for 3 Target Durations
• I-Frame Required to Start Playback
• Full Segment Required after All Parts
• New Packagers Required
• Can we Share CDN Cache (between Parts & Segments)
− HTTP/1.1 Chunked Transfer from Origin to Edge
− Query Params for Byte Ranges on Edge
(i.e. filePart271.ts?start=0&end=123)

13. 1 - Partial Segments (Challenges)
• Can we Share CDN Cache (between Parts & Segments)
− HTTP/1.1 Chunked Transfer from Origin to Edge
− Query Params for Byte Ranges on Edge
(i.e. filePart271.ts?start=0&end=123)
#EXT-X-PART:DURATION=0.33334,URI="filePart271.ts?start=0&end=123"
#EXT-X-PART:DURATION=0.33334,URI="filePart271.ts?start=124&end=456"
...
#EXTINF:4.00008,
filePart271.ts

14. 2 - Reducing Latency of Segments
• Mobile Networks can have high Round Trip Times
• HTTP/2 Server Push for Parts
− Triggered alongside Manifest
− _HLS_push to Indicate Client expects part to be pushed
• Link Response Header
− Link: </filePart.456.1.ts>; rel=preload; as=video; type=video/mp2t
• Challenges
− CDN Support Varies
− Manifests & Segments on Different Domains/Origins

15. 3- Blocking Playlist Requests
• Today
− Client Polls Manifest for New Segments
• Blocking Request for Next Playlist
− _HLS_msn for Media Sequence Number
− _HLS_part for Part Number
• CDN should Batch Requests for Same Playlist

16. 4 - Playlist Delta Updates
• Today
− Playlists Keep Getting Larger
• More Frequent Manifest Requests with Low Latency
• First Playlist Request as Normal
• New #EXT-X-SKIP tag
− Indicates number of Skipped Segments
• _HLS_skip=YES used on Origin API

17. #EXTM3U
# Following the example above, this playlist is a response to: GET
https://example.com/2M/waitForMSN.php?_HLS_msn=273&_HLS_part=3&_HLS_report=../1M/waitForMSN.php&_HLS_report=../4M/waitForMSN.php&_HLS_skip=YES
#EXT-X-TARGETDURATION:4
#EXT-X-VERSION:9
#EXT-X-SERVER-CONTROL:CAN-BLOCK-RELOAD=YES,PART-HOLD-BACK=1.0,CAN-SKIP-UNTIL=12.0
#EXT-X-PART-INF:PART-TARGET=0.33334
#EXT-X-MEDIA-SEQUENCE:266
#EXT-X-SKIP:SKIPPED-SEGMENTS=3
#EXTINF:4.00008,
fileSequence269.ts
#EXTINF:4.00008,
fileSequence270.ts
#EXT-X-PART:DURATION=0.33334,URI="filePart271.0.ts",INDEPENDENT=YES
#EXT-X-PART:DURATION=0.33334,URI="filePart271.1.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart271.2.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart271.3.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart271.4.ts",INDEPENDENT=YES
#EXTINF:4.00008,
fileSequence271.ts
#EXT-X-PROGRAM-DATE-TIME:2019-02-14T02:13:60.106Z
#EXT-X-PART:DURATION=0.33334,URI="filePart272.a.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart272.b.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart272.c.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart272.d.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart272.e.ts"
#EXT-X-PART:DURATION=0.33334,URI="filePart272.f.ts",INDEPENDENT=YES
#EXT-X-RENDITION-REPORT:URI="../1M/waitForMSN.php",LAST-MSN=272,LAST-PART=11
#EXT-X-RENDITION-REPORT:URI="../4M/waitForMSN.php",LAST-MSN=272,LAST-PART=11

18. 5 - Faster Rendition Switches
• Less Time to React with Low Latency
• Include URI to Next Segments of Different Renditions
• Adds #EXT-X-RENDITION-REPORT Tag
• #EXT-X-RENDITION-REPORT:URI="../1M/waitForMSN.php",LAST-MSN=273,LAST-PART=2
• #EXT-X-RENDITION-REPORT:URI="../4M/waitForMSN.php",LAST-MSN=273,LAST-PART=1
• Spec Mentions LAST-I-MSN and LAST-I-PART Unused
• How do we determine which Part we can Switch on?
− Is it safe to use INDEPENDENT from Current Renditions
Parts?

19. Conclusion
• A LOT of Moving Pieces
• Origin API adds Complexity
− For Client, CDN & Origin
• Origins Must Become More Dynamic
• Lots of Unknowns for SSAI Integration/Support
• HTTP/2 & Server Push has Varying Levels of Support
• Bypasses Bandwidth Estimation Issues of Other Approaches

20. Conclusion
• In Beta in iOS 13 Beta behind Entitlement
− No Safari Support Yet
• Sample Script / Tools Provided by Apple
• Will this be New Standard for LHLS?

21. THANK
YOU