104 lines
4.2 KiB
ReStructuredText
104 lines
4.2 KiB
ReStructuredText
####################################
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Low Latency DASH (LL-DASH) Streaming
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####################################
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************
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Introduction
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************
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If ``--low_latency_dash_mode`` is enabled, low latency DASH (LL-DASH) packaging will be used.
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This will reduce overall latency by ensuring that the media segments are chunk encoded and delivered via an aggregating response.
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The combination of these features will ensure that overall latency can be decoupled from the segment duration.
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For low latency to be achieved, the output of Shaka Packager must be combined with a delivery system which can chain together a set of aggregating responses, such as chunked transfer encoding under HTTP/1.1 or a HTTP/2 or HTTP/3 connection.
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The output of Shaka Packager must be played with a DASH client that understands the availabilityTimeOffset MPD value.
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Furthermore, the player should also understand the throughput estimation and ABR challenges that arise when operating in the low latency regime.
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This tutorial covers LL-DASH packaging and uses features from the DASH, HTTP upload, and FFmpeg piping tutorials.
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For more information on DASH, see :doc:`dash`; for HTTP upload, see :doc:`http_upload`;
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for FFmpeg piping, see :doc:`ffmpeg_piping`;
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for full documentation, see :doc:`/documentation`.
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*************
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Documentation
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*************
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Getting started
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===============
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To enable LL-DASH mode, set the ``--low_latency_dash_mode`` flag to ``true``.
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All HTTP requests will use chunked transfer encoding:
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``Transfer-Encoding: chunked``.
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.. note::
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Only LL-DASH is supported. LL-HLS support is yet to come.
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Synopsis
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========
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Here is a basic example of the LL-DASH support.
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The LL-DASH setup borrows features from "FFmpeg piping" and "HTTP upload",
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see :doc:`ffmpeg_piping` and :doc:`http_upload`.
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Define UNIX pipe to connect ffmpeg with packager::
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export PIPE=/tmp/bigbuckbunny.fifo
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mkfifo ${PIPE}
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Acquire and transcode RTMP stream::
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ffmpeg -fflags nobuffer -threads 0 -y \
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-i rtmp://184.72.239.149/vod/mp4:bigbuckbunny_450.mp4 \
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-pix_fmt yuv420p -vcodec libx264 -preset:v superfast -acodec aac \
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-f mpegts pipe: > ${PIPE}
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Configure and run packager::
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# Define upload URL
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export UPLOAD_URL=http://localhost:6767/ll-dash
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# Go
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packager \
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"input=${PIPE},stream=audio,init_segment=${UPLOAD_URL}_init.m4s,segment_template=${UPLOAD_URL}/bigbuckbunny-audio-aac-\$Number%04d\$.m4s" \
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"input=${PIPE},stream=video,init_segment=${UPLOAD_URL}_init.m4s,segment_template=${UPLOAD_URL}/bigbuckbunny-video-h264-450-\$Number%04d\$.m4s" \
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--io_block_size 65536 \
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--segment_duration 2 \
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--low_latency_dash_mode=true \
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--utc_timings "urn:mpeg:dash:utc:http-xsdate:2014"="https://time.akamai.com/?iso" \
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--mpd_output "${UPLOAD_URL}/bigbuckbunny.mpd" \
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*************************
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Low Latency Compatibility
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*************************
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For low latency to be achieved, the processes handling Shaka Packager's output, such as the server and player,
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must support LL-DASH streaming.
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Delivery Pipeline
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=================
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Shaka Packager will upload the LL-DASH content to the specified output via HTTP chunked transfer encoding.
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The server must have the ability to handle this type of request. If using a proxy or shim for cloud authentication,
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these services must also support HTTP chunked transfer encoding.
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Examples of supporting content delivery systems:
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* `AWS MediaStore <https://aws.amazon.com/mediastore/>`_
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* `s3-upload-proxy <https://github.com/fsouza/s3-upload-proxy>`_
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* `Streamline Low Latency DASH preview <https://github.com/streamlinevideo/low-latency-preview>`_
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* `go-chunked-streaming-server <https://github.com/mjneil/go-chunked-streaming-server>`_
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Player
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======
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The player must support LL-DASH playout.
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LL-DASH requires the player to be able to interpret ``availabilityTimeOffset`` values from the DASH MPD.
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The player should also recognize the the throughput estimation and ABR challenges that arise with low latency streaming.
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Examples of supporting players:
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* `Shaka Player <https://github.com/shaka-project/shaka-player>`_
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* `dash.js <https://github.com/Dash-Industry-Forum/dash.js>`_
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* `Streamline Low Latency DASH preview <https://github.com/streamlinevideo/low-latency-preview>`_
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