Comment by urlgrey | Hacker News Reader

Comment by urlgrey | Hacker News Reader

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Periscope developed a Low-Latency HTTP Live Streaming (LHLS) technique that relies on HTTP chunked transfer-encoding to stream video bytes as they are encoded at the origin. This is still subject to TCP packet retransmission overhead, but the time-to-first-byte is reduced significantly and leads to less buffering on the client.

Here's a Periscope post about LHLS: https://medium.com/@periscopecode/introducing-lhls-media-str...

Most systems that serve HLS media use fixed content-length segments, which requires knowledge of the length of a segment before the first byte can be sent over the wire. So, for a 5 second segment you would need to encode the entire 5 seconds before the first byte can be sent; this does not apply when streaming the segments with chunked transfer encoding.

Incidentally, at Mux we also use chunked transfer-encoding to stream video that is encoded on-the-fly with great performance.

I've heard from colleagues that this won't be possible with DASH due to the switch to fMP4 format. One of my co-workers tells me that fMP4 requires the entire segment to be loaded before playback can begin while TS segments don't require this. We've been looking into very small segments (e.g. 1s duration) to reduce latency but I've been interested in the LHLS approach since I first heard of it.

> I've heard from colleagues that this won't be possible with DASH due to the switch to fMP4 format.

That's incorrect. With DASH the latency depends on the fragment duration, not the segment duration. You can start sending the segment when its first fragment is generated, and use chunk-based HTTP transfer as mentioned in other comments.

Link for further details on low latency: https://www.gpac-licensing.com/2014/07/09/lowering-dash-live...

Very short segment durations are effective only when latency is more important than quality.

Each TS segment must start with a key-frame, and the GOP size can't exceed the duration of a segment (e.g. one second). Lowering the segment duration increases the frequency of key-frames, which has the effect of lowering the quality you can achieve at a given bitrate.

Note that this is a Apple requirement for HLS. Most people don't realize that the GOP size doesn't impact the latency, but it impacts start-up time.

fMP4 has an index for each chunk, so you have to buffer the whole thing to create the index on the writing side. However, with DASH you do have the option for WebM as well, which does not need and index and can be streamed. Or really small fMP4 segments work too.

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