[Hls-interest] iOS 17 and Low-Latency HLS

[Roger Pantos <[e-mail]>]

Hi folks. Now that we've shipped the second (developer) seed of iOS 17, there are a couple of bits of news I'd like to share.

First: as many LL-HLS aficionados are aware, Appendix B.1 of the spec (Low-Latency Server Configuration Profile) says that LL-HLS must be delivered over HTTP/2. Meanwhile, HTTP/3 (IETF QUIC) is conspicuously absent. In iOS 17 we are adding support for LL-HLS delivery over HTTP/3, beginning with seed 2.

Because H3 came with a new stream prioritization scheme, the spec will be updated to accommodate it. Exact language is forthcoming, but the basic upshot is that LL-HLS will require (on H3) or strongly recommend (on H2) the use of static, server-defined priorities described by RFC 9218 (Extensible Prioritization Scheme for HTTP). Playlists must be delivered at priority 1 while (partial) segments must be in the lower-priority band from 2 to 6, with higher bit rate renditions having equal or lower priority than lower bit rate renditions. These priorities must be signaled in the H3 response headers, or iOS 17 will play the stream in regular-latency mode.

Second: we've done a bunch of tuning on the LL-HLS ABR in iOS 17, aimed at improving its performance in difficult network conditions. We expect significantly lower stall rates compared to earlier releases while maintaining (or in some cases improving) the average bit rates at which the content is played. We encourage everyone who is experimenting with LL-HLS to try their streams on iOS 17, and to let us know what they find. (Hopefully we didn't break anything, but if we did there is still a small amount of time to fix things before iOS 17 goes live.)

One last note: for best results, test your LL-HLS streams on iOS 17 using a native app, not through Safari. While Safari supports LL-HLS, the lengths to which it must go to make resource loading HTML5-compliant impacts streaming performance, particularly at low latencies.


cheers,

Roger Pantos
Apple Inc.