lowly

Low-latency live streaming proof-of-concept. Attempts to implement the server portion of Apple's low-latency extensions to HLS.

Deployment overview

Input:

• Transport Stream
• over RTP
• on UDP port 5000

Output:

• HTTP1.1 (add HTTP2 support with a proxy like Nginx)
• on TCP port 5050
• Master manifest published at /master.m3u8

[encoder]  --MPEGTS/RTP-->  [lowly]  --HLS/HTTP1.1-->  [nginx]  --HLS/HTTP2-->  [player]

⛔ Limitations

• Hardcoded rewind window (1 hour)
• In-memory only! Media is not written to persistent storage.
• Audio must be AAC 48khz
• Video must be AVC 25fps, with an IDR every 48 frames
• Haven't been able to test the low latency aspect on an actual player! (Standard latency stream has had basic tests on Safari / iOS.)
• prompeg FEC support on input planned but not available yet

Feature support

Low latency extensions,

• Partial segments (EXT-X-PART)
• Blocking media-manifest reloads (_HLS_msn / _HLS_part support)
• HTTP2 push of parts (if using an HTTP2 reverse proxy supporting Link: .. rel=preload header, like Nginx)
• Playlist Delta Updates (EXT-X-SKIP / _HLS_skip=YES not yet supported)
• Rendition reports (EXT-X-RENDITION-REPORT / _HLS_report not yet supported)

General HLS features,

• No ABR! (can only ingest a single audio stream and a single video stream right now)
• Codecs
  • AVC
  • AAC
  • no others supported (no HEVC etc)
• fMP4 segments
• TS segments unsupported
• BANDWIDTH signalling (supported via maximum_bitrate_descriptor in input)
• No FRAMERATE (planned via inspection of HEVC headers)
• No captions / subtitles
• No SCTE signalling
• No language code signalling (planned via language_descriptor metadata in input)
• No EXT-X-DISCONTINUITY signalling (if the input has a discontinuity, the output will be invalid HLS)
• EXT-X-PROGRAM-DATE-TIME (if AVC pic_timing metadata is in the source stream)
• No EXT-X-I-FRAME-STREAM-INF / EXT-X-I-FRAMES-ONLY
• No DRM
• No EXT-X-ENDLIST (there's currently no way to end the stream)
• EXT-X-MEDIA-SEQUENCE (after stream duration reaches hardcoded limit and old segments start being removed)
• doubtless lots of other mandatory spec features that are not implemented right now!

See also

https://en.wikipedia.org/wiki/Lowly_Worm