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MPD Patch details #437

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165 changes: 157 additions & 8 deletions specs/mpd-patch/01-mpd-patch.inc.md
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## Introduction ## {#mpd-patch_introduction}

This feature description is an update to DASH-IF IOP Guidelines v4.3 [[!IOP43]].
It adds an additional features.
It adds an additional feature, the MPD Patch, a way provide a differential
update to a previous dynamic MPD.

## Scenarios ## {#mpd-patch_scenarios}

The typical use case for MPD patch is for dynamic manifests with long
SegmentTimeline elements due to non-constant segment duration prohibiting
efficient compression using repeat count.
This happens when the media frame rate is not commensurable with
the segment duration.

For example, a 2s average segment duration is not compatible with AAC
48kHz audio, since it corresponds to 93.75 frames leading to a cycle
of 4 segments (8s) and the corresponding SegmentTimline pattern:

```xml
<SegmentTimeline>
<S t="83498463744000" d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
<S d="95232"></S>
<!-- continued pattern -->
</SegmentTimeline>
```

For a long sliding window, this results in a huge MPD. With MPD Patch
one can instead request a delta document, a Patch, describing the changes
relative to a `publishTime`.

## Content Offering Requirements and Recommendations ## {#mpd-patch_content-offering}

MPD Patch is essentially only useful in the case of `SegmentTemplate with
SegmentTimeline`. It is especially useful when the segment durations are
varying leading to long `SegmentTimeline` nodes. It may also help in the
case of multiple periods of a dynamic MPD.

MPD Patch should NOT be used for for `SegmentTemplate with $Number$` since
such MPDs typically rarely change, meaning that their content including the
`publishTime` is the same over a longer period of time.

The `PatchLocation` element in the MPD contains an optional `ttl` attribute
providing the availability end time relative to the `publishTime`.
It is recommended to use this value, and set it to relatively small number
like 1 minute.

The node serving the MPD Patch requests can cache the first response
with an updated `publishTime` respect to the referred one, provided
that the time difference is less than the `ttl` value.

A client can therefore NOT assume that the Patch response is providing
information about the latest publishTime. It follows that he client may need
to make more MPD Patch requests to arrive at the live edge.

The server response to a too early request for an MPD Patch,
i.e. before there is a new publishTime, should be the same as when
asking for a segment before its availability time. That could be
`404 Not Found` or `425 Too Early`.

## Client Implementation Requirements and Guidelines ## {#mpd-patch_client}

Clients should ignore the `<PatchLocation>` element if not understood.
If used, they should make a request for an Patch at the same instant
that they would ask for an updated MPD.

If they get a `4XX` response to the Patch request, they should either
wait and redo the request, or switch to fetching a full MPD.

## Examples ## {#mpd-patch_examples}

NOTE_ Add some MPD examples
Below is an example with a `<PatchLocation>` element and
`publishTime="2024-04-16T07:34:38Z`.

```xml
<?xml version="1.0" encoding="UTF-8"?>
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:mpeg:dash:schema:mpd:2011 DASH-MPD.xsd" id="auto-patch-id"
profiles="urn:mpeg:dash:profile:isoff-live:2011,http://dashif.org/guidelines/dash-if-simple"
type="dynamic" availabilityStartTime="1970-01-01T00:00:00Z" publishTime="2024-04-16T07:34:38Z" minimumUpdatePeriod="PT2S"
minBufferTime="PT2S" timeShiftBufferDepth="PT1M" maxSegmentDuration="PT2S">
<ProgramInformation moreInformationURL="https://github.com/dash-Industry-Forum/livesim-content">
<Title>Basic MPD with 640x480@30 video at 300kbp and 48kbps audio</Title>
<Source>VoD source for DASH-IF livesim2</Source>
</ProgramInformation>
<PatchLocation ttl="60">/patch/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpp?publishTime=2024-04-16T07%3A34%3A38Z</PatchLocation>
<Period id="P0" start="PT0S">
<AdaptationSet id="2" lang="en" contentType="audio" segmentAlignment="true" mimeType="audio/mp4" startWithSAP="1">
<Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"></Role>
<SegmentTemplate media="$RepresentationID$/$Time$.m4s" initialization="$RepresentationID$/init.mp4" timescale="48000">
<SegmentTimeline>
<S t="82236135168000" d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
<S d="95232"></S>
<S d="96256" r="2"></S>
</SegmentTimeline>
</SegmentTemplate>
<Representation id="A48" bandwidth="48000" audioSamplingRate="48000" codecs="mp4a.40.2">
<AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"></AudioChannelConfiguration>
</Representation>
</AdaptationSet>
<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">
<Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"></Role>
<SegmentTemplate media="$RepresentationID$/$Time$.m4s" initialization="$RepresentationID$/init.mp4" timescale="90000">
<SegmentTimeline>
<S t="154192753440000" d="180000" r="30"></S>
</SegmentTimeline>
</SegmentTemplate>
<Representation id="V300" bandwidth="300000" width="640" height="360" sar="1:1" frameRate="60/2" codecs="avc1.64001e"></Representation>
</AdaptationSet>
</Period>
<UTCTiming schemeIdUri="urn:mpeg:dash:utc:http-xsdate:2014" value="https://time.akamai.com/?iso&amp;ms"></UTCTiming>
</MPD>
```

The segments have an average duration of 2s, so a request 5s after the publishTime
results in the following PATCH document with a more complex change
to the audio part than to the video part.

```xml
<?xml version="1.0" encoding="UTF-8"?>
<Patch xmlns="urn:mpeg:dash:schema:mpd-patch:2020"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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">
<replace sel="/MPD/@publishTime">2024-04-16T07:34:42Z</replace>
<replace sel="/MPD/PatchLocation[1]">
<PatchLocation ttl="60">/patch/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpp?publishTime=2024-04-16T07%3A34%3A42Z</PatchLocation>
</replace>
<remove sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline/S[1]"/>
<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline" pos="prepend">
<S t="82236135360512" d="96256"/>
</add>
<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline/S[15]" pos="after">
<S d="95232"/>
</add>
<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='2']/SegmentTemplate/SegmentTimeline/S[16]" pos="after">
<S d="96256"/>
</add>
<remove sel="/MPD/Period[@id='P0']/AdaptationSet[@id='1']/SegmentTemplate/SegmentTimeline/S[1]"/>
<add sel="/MPD/Period[@id='P0']/AdaptationSet[@id='1']/SegmentTemplate/SegmentTimeline" pos="prepend">
<S t="154192753800000" d="180000" r="30"/>
</add>
</Patch>
```

## Reference Tools ## {#mpd-patch_reference-tools}

NOTE_ provide status for the following functionalities
* Dash.js
* Live Sim
* Test Vectors
NOTE: provide status for the following functionalities

* dash.js supports MPD Patch for `SegmentTemplate with SegmentTimeline`.
It has been tested towards [livesim2][livesim2] including cases with
multiple periods. If a 4XX response is received, it will switches to
ordinary full MPD requests (Daniel to confirm)
* [livesim2][livesim2] supports MPD DASH. There is [Wiki article][livesim2-wiki]
describing how it works
* Test Vectors. One can get test vectors from the
[DASH-IF instance of livesim2][livesim2-instance], e.g.
[https://livesim2.dashif.org/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpd][livesim2-entry].
Use the [urlgen][urlgen] page to generate other test vectors.
* JCCP

## Additional Information ## {#mpd-patch_additional-information}



[livesim2]: https://github.com/Dash-Industry-Forum/livesim2
[livesim2-wiki]: https://github.com/Dash-Industry-Forum/livesim2/wiki/MPD-Patch
[livesim2-instance]: https://livesim2.dashif.org
[livesim2-entry]:https://livesim2.dashif.org/livesim2/patch_60/segtimeline_1/testpic_2s/Manifest.mpd
[urlgen]: https://livesim2.dashif.org/urlgen