Re: [Hls-interest] HLS Content Steering v1.0b1 preliminary specification

[Eryk Vershen <[e-mail]>]

I see some issues with the approach you suggest.

> On Feb 17, 2021, at 1:46 PM, Pieter-Jan Speelmans <[e-mail]> wrote:
> 
> Hi all
> 
> I am pretty concerned about the backwards compatibility problems when attempting to use this in combination with older devices. It will force services to either provide two playlists, one with and one without EXT-X-DEFINE with client-side detection of the maximum version supported by the client. While this will not be an issue for up to date Apple devices, this will definitely be an issue for out of date devices but also most smart TVs, Chromecasts, Airplay implementations by those other than Apple (f.e. on LG), ...
> 
> One option which could avoid this would be the following approach:
> - Leverage the STABLE-VARIANT-ID and STABLE-RENDITION-ID identifiers to modify an attribute of the Variant or Rendition.

This suggests being able to modify attributes other than the URI. We explicitly disallow that.
Allowing non-URI attributes to be modified creates the possibility for the structure of the master playlist to be changed significantly.
This creates major problems for the player.


> - Allow a Variant or Rendition to be referenced in the Steering Manifest with an updated value for the relevant attribute.

This effectively means there are two mechanisms for substituting values: the existing variable substitution (EXT-X-DEFINE) and URI substitution via steering. Two mechanisms doing similar things is confusing. In addition, we would need to work out whether (and how) the two mechanisms could be used in the same playlist.


> - Give the EXT-X-CONTENT-STEERING tag a MODIFIES attribute to list the Variants and Renditions which will be modified and VARIANT-ATTRIBUTES and RENDITION-ATTRIBUTES attributes to list the attributes which can be modified rather than the DEFINES attribute.
> 
> The language for a Pathway would be defined as:
> {
>     "ID": string, // REQUIRED
>     "VARIANTS": { // REQUIRED
>         "<STABLE-VARIANT-ID>": { // one for every variant listed to be modified
>             Key-Value pairs for every attribute which you want to redefine
>         }
>     },
>     "RENDITIONS": { // OPTIONAL 
>         "<STABLE-RENDITION-ID>": {  // one for every rendition listed as to be modified
>             Key-Value pairs for every attribute which you want to redefine
>         } 
>     } 
> }

This is likely to increase the size of the manifests. A playlist can have several dozen stable iDs. That makes the notion of using a data URI as the SERVER-URI more problematic.


> For example this could make the Master Playlist.
> #EXTM3U
> #EXT-X-VERSION:8
> 
> #EXT-X-CONTENT-STEERING:MODIFIED="VARIANT-1,VARIANT-2",VARIANT-ATTRIBUTES="URI",SERVER-URI="/steering?video=00001",PATHWAY-ID="My-Great-CDN"
>  
> #EXT-X-STREAM-INF:BANDWIDTH=1280000,CODECS="avc1.640028",STABLE-VARIANT-ID="VARIANT-1"
> https://cdn-a.example.com/low/main/audio-video.m3u8 <https://cdn-a.example.com/low/main/audio-video.m3u8>
> #EXT-X-STREAM-INF:BANDWIDTH=7680000,CODECS="avc1.640028",STABLE-VARIANT-ID="VARIANT-2"
> https://cdn-a.example.com/hi/main/audio-video.m3u8 <https://cdn-a.example.com/hi/main/audio-video.m3u8>
> 
> Which results into a Steering Manifest like the one below.
> {
>     "TTL": 300,
>     "RELOAD-URI": "https://example.com/steering?video=00001&session=123 <https://example.com/steering?video=00001&session=123>",
>     "PATHWAYS": [
>         {
>             "ID": "My-Great-CDN",
>             "VARIANTS": {
>                 "VARIANT-1": {
>                     "URI": "https://cdn-a.example.com/low/main/audio-video.m3u8 <https://cdn-a.example.com/low/main/audio-video.m3u8>"
>                 } 
>                 "VARIANT-2":{
>                     "URI": "https://cdn-a.example.com/hi/main/audio-video.m3u8 <https://cdn-a.example.com/hi/main/audio-video.m3u8>"
>                 } 
>             }
>         },
>         {
>             "ID": "My-Backup-CDN",
>             "VARIANTS": {
>                 "VARIANT-1":{
>                     "URI": "https://cdn-b.example.com/low/main/audio-video.m3u8 <https://cdn-b.example.com/low/main/audio-video.m3u8>"
>                 } 
>                 "VARIANT-2":{
>                     "URI": "https://cdn-b.example.com/hi/main/audio-video.m3u8 <https://cdn-b.example.com/hi/main/audio-video.m3u8>"
>                 } 
>             }
>         }
>     ]
> }

With this design, the steering manifest is effectively required to be "per asset" rather than shareable across many assets.

Regards,
-eryk

Eryk Vershen
Apple Inc.


> Benefits:
> - Fully backwards compatible. Older clients will simply ignore the new EXT-X-CONTENT-STEERING tag, and use the primary source.
> - No change needed to the Master Playlist other than the addition of the #EXT-X-CONTENT-STEERING tag.
> - Also compatible with the AUTH-case for EXT-X-DEFINE by allowing the Variable Substitution on attributes after retrieval from the Steering Manifest 
> - As an added bonus, for those who are modifying the master playlist anyway, the use of STABLE-VARIANT-ID and STABLE-RENDITION-ID allows clients to identify and identify the Variants and Renditions.
> 
> Drawback:
> - More closely coupled with HLS, making reuse of the Steering Manifest for other failover cases less likely.
> 
> My personal opinion is that the value of backwards compatibility would warrant the drawbacks I currently see. 
> 
> Best regards,
> Pieter-Jan
> 
> 
> Pieter-Jan Speelmans
> CTO
> 
> THEO Technologies NV
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> 
> On Wed, Feb 17, 2021 at 9:57 PM Roger Pantos <rpantos=[e-mail] <mailto:[e-mail]>> wrote:
> Hello hls-interest,
> 
> As discussed on the call today, here is the preliminary specification for the new Content Steering feature. We are converging on the final design for 1.0, so please take a look at it when you can and tell us what you think. 
> 
> You can send feedback to this list, or to [e-mail] <mailto:[e-mail]> if you want it to go directly to us.
> 
> 
> 
> 
> regards,
> 
> Roger Pantos
> Apple Inc.-- 
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