[Hls-interest] Reducing latency when TARGETDURATION is conservative

[Luke Curley <[e-mail]>]

Hey folks,

We're trying to reduce HLS latency for iOS users (AVPlayer) but we're
running into a few issues.

We don't transcode all streams, so the broadcaster is often responsible for
setting the size of GOPs. While most broadcasting software will use a fixed
GOP interval, like 2s for OBS, it's not a guarantee. We set a conservative
value of 6s for TARGETDURATION just in case.

>From our testing, 2s segments and 6s TARGETDURATION results in 18s+ latency
with AVPlayer. The player seems to choose 3 * TARGETDURATION as the initial
latency, which is quite excessive for our use-case.

The latest HLS RFC adds the HOLD-BACK attribute
<https://tools.ietf.org/html/draft-pantos-hls-rfc8216bis-08#section-4.4.3.8>,
allowing the server to choose the initial latency. Unfortunately, this
attribute "MUST be at least three times the Target Duration" so it only
confirms the AVPlayer behavior. I think this requirement could be relaxed
without any negative impact.

There's still quite a lot of work remaining on our end to support LL-HLS.
It would be great if we could reduce HLS latency while working on adding
full LL-HLS support (ex. sub-segments, HTTP/2, etc).


Thanks!