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Merge pull request #437 from Dash-Industry-Forum/mpd-patch-description
MPD Patch details
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‎specs/mpd-patch/01-mpd-patch.inc.md‎

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## Introduction ## {#mpd-patch_introduction}
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This feature description is an update to DASH-IF IOP Guidelines v4.3 [[!IOP43]].
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It adds an additional features.
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It adds an additional feature, the MPD Patch, a way provide a differential
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update to a previous dynamic MPD.
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## Scenarios ## {#mpd-patch_scenarios}
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The typical use case for MPD patch is for dynamic manifests with long
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SegmentTimeline elements due to non-constant segment duration prohibiting
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efficient compression using repeat count.
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This happens when the media frame rate is not commensurable with
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the segment duration.
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For example, a 2s average segment duration is not compatible with AAC
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48kHz audio, since it corresponds to 93.75 frames leading to a cycle
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of 4 segments (8s) and the corresponding SegmentTimline pattern:
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```xml
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<SegmentTimeline>
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<S t="83498463744000" d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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<S d="95232"></S>
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<!-- continued pattern -->
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</SegmentTimeline>
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```
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For a long sliding window, this results in a huge MPD. With MPD Patch
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one can instead request a delta document, a Patch, describing the changes
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relative to a `publishTime`.
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## Content Offering Requirements and Recommendations ## {#mpd-patch_content-offering}
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MPD Patch is essentially only useful in the case of `SegmentTemplate with
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SegmentTimeline`. It is especially useful when the segment durations are
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varying leading to long `SegmentTimeline` nodes. It may also help in the
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case of multiple periods of a dynamic MPD.
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MPD Patch should NOT be used for for `SegmentTemplate with $Number$` since
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such MPDs typically rarely change, meaning that their content including the
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`publishTime` is the same over a longer period of time.
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The `PatchLocation` element in the MPD contains an optional `ttl` attribute
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providing the availability end time relative to the `publishTime`.
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It is recommended to use this value, and set it to relatively small number
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like 1 minute.
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The node serving the MPD Patch requests can cache the first response
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with an updated `publishTime` respect to the referred one, provided
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that the time difference is less than the `ttl` value.
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A client can therefore NOT assume that the Patch response is providing
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information about the latest publishTime. It follows that he client may need
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to make more MPD Patch requests to arrive at the live edge.
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The server response to a too early request for an MPD Patch,
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i.e. before there is a new publishTime, should be the same as when
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asking for a segment before its availability time. That could be
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`404 Not Found` or `425 Too Early`.
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## Client Implementation Requirements and Guidelines ## {#mpd-patch_client}
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Clients should ignore the `<PatchLocation>` element if not understood.
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If used, they should make a request for an Patch at the same instant
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that they would ask for an updated MPD.
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If they get a `4XX` response to the Patch request, they should either
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wait and redo the request, or switch to fetching a full MPD.
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## Examples ## {#mpd-patch_examples}
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NOTE_ Add some MPD examples
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Below is an example with a `<PatchLocation>` element and
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`publishTime="2024-04-16T07:34:38Z`.
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```xml
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<?xml version="1.0" encoding="UTF-8"?>
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<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="urn:mpeg:dash:schema:mpd:2011 DASH-MPD.xsd" id="auto-patch-id"
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profiles="urn:mpeg:dash:profile:isoff-live:2011,http://dashif.org/guidelines/dash-if-simple"
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type="dynamic" availabilityStartTime="1970-01-01T00:00:00Z" publishTime="2024-04-16T07:34:38Z" minimumUpdatePeriod="PT2S"
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minBufferTime="PT2S" timeShiftBufferDepth="PT1M" maxSegmentDuration="PT2S">
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<ProgramInformation moreInformationURL="https://github.com/dash-Industry-Forum/livesim-content">
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<Title>Basic MPD with 640x480@30 video at 300kbp and 48kbps audio</Title>
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<Source>VoD source for DASH-IF livesim2</Source>
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</ProgramInformation>
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<PatchLocation ttl="60">/patch/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpp?publishTime=2024-04-16T07%3A34%3A38Z</PatchLocation>
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<Period id="P0" start="PT0S">
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<AdaptationSet id="2" lang="en" contentType="audio" segmentAlignment="true" mimeType="audio/mp4" startWithSAP="1">
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<Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"></Role>
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<SegmentTemplate media="$RepresentationID$/$Time$.m4s" initialization="$RepresentationID$/init.mp4" timescale="48000">
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<SegmentTimeline>
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<S t="82236135168000" d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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<S d="95232"></S>
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<S d="96256" r="2"></S>
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</SegmentTimeline>
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</SegmentTemplate>
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<Representation id="A48" bandwidth="48000" audioSamplingRate="48000" codecs="mp4a.40.2">
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<AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"></AudioChannelConfiguration>
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</Representation>
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</AdaptationSet>
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<AdaptationSet id="1" contentType="video" par="16:9" minWidth="640" maxWidth="640" minHeight="360" maxHeight="360" maxFrameRate="60/2" segmentAlignment="true" mimeType="video/mp4" startWithSAP="1">
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<Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"></Role>
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<SegmentTemplate media="$RepresentationID$/$Time$.m4s" initialization="$RepresentationID$/init.mp4" timescale="90000">
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<SegmentTimeline>
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<S t="154192753440000" d="180000" r="30"></S>
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</SegmentTimeline>
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</SegmentTemplate>
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<Representation id="V300" bandwidth="300000" width="640" height="360" sar="1:1" frameRate="60/2" codecs="avc1.64001e"></Representation>
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</AdaptationSet>
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</Period>
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<UTCTiming schemeIdUri="urn:mpeg:dash:utc:http-xsdate:2014" value="https://time.akamai.com/?iso&amp;ms"></UTCTiming>
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</MPD>
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```
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The segments have an average duration of 2s, so a request 5s after the publishTime
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results in the following PATCH document with a more complex change
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to the audio part than to the video part.
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```xml
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<?xml version="1.0" encoding="UTF-8"?>
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<Patch xmlns="urn:mpeg:dash:schema:mpd-patch:2020"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="urn:mpeg:dash:schema:mpd-patch:2020 DASH-MPD-PATCH.xsd" mpdId="auto-patch-id" originalPublishTime="2024-04-16T07:34:38Z" publishTime="2024-04-16T07:34:42Z">
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<replace sel="/MPD/@publishTime">2024-04-16T07:34:42Z</replace>
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<replace sel="/MPD/PatchLocation[1]">
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<PatchLocation ttl="60">/patch/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpp?publishTime=2024-04-16T07%3A34%3A42Z</PatchLocation>
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</replace>
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<remove sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline/S[1]"/>
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<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline" pos="prepend">
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<S t="82236135360512" d="96256"/>
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</add>
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<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline/S[15]" pos="after">
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<S d="95232"/>
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</add>
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<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline/S[16]" pos="after">
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<S d="96256"/>
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</add>
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<remove sel="/MPD/Period[@id='P0']/AdaptationSet[@id='1']/SegmentTemplate/SegmentTimeline/S[1]"/>
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<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='1']/SegmentTemplate/SegmentTimeline" pos="prepend">
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<S t="154192753800000" d="180000" r="30"/>
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</add>
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</Patch>
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```
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## Reference Tools ## {#mpd-patch_reference-tools}
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NOTE_ provide status for the following functionalities
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* Dash.js
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* Live Sim
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* Test Vectors
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NOTE: provide status for the following functionalities
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* dash.js supports MPD Patch for `SegmentTemplate with SegmentTimeline`.
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It has been tested towards [livesim2][livesim2] including cases with
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multiple periods. If a 4XX response is received, it will switches to
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ordinary full MPD requests (Daniel to confirm)
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* [livesim2][livesim2] supports MPD DASH. There is [Wiki article][livesim2-wiki]
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describing how it works
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* Test Vectors. One can get test vectors from the
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[DASH-IF instance of livesim2][livesim2-instance], e.g.
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[https://livesim2.dashif.org/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpd][livesim2-entry].
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Use the [urlgen][urlgen] page to generate other test vectors.
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* JCCP
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## Additional Information ## {#mpd-patch_additional-information}
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[livesim2]: https://github.com/Dash-Industry-Forum/livesim2
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[livesim2-wiki]: https://github.com/Dash-Industry-Forum/livesim2/wiki/MPD-Patch
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[livesim2-instance]: https://livesim2.dashif.org
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[livesim2-entry]:https://livesim2.dashif.org/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpd
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[urlgen]: https://livesim2.dashif.org/urlgen

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