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

["Law, Will" <[e-mail]>]

Hi Roger

One comment here around the inconsistency of the ‘prefix’ rules between the two cases. The player is being asked to perform two different operations, with “” being a special flag that differentiates between the two.  I will explain the problem and then propose a solution.

The current master playlist #EXT-X-STEERING-TEMPLATE attribute is called “prefix”, which implies something inserted ahead of something else. However if we evaluate the first case with an absolute URL

#EXTM3U
#EXT-X-CONTENT-STEERING:SERVER-URI="https://example.com/steering?video=00001<https://urldefense.com/v3/__https:/example.com/steering?video=00001&session=123__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qtYzaA4$>",PATHWAY-ID="My-Great-CDN"
#EXT-X-STEERING-TEMPLATE:ID="1",PREFIX="https://a.example.com/video<https://urldefense.com/v3/__https:/a.example.com/video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mqwdeF4$>"
#EXT-X-STREAM-INF:BANDWIDTH=200000,STEERING="1"
https://a.example.com/video/my-movie/mid/index.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/mid/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44hforWWE$>
#EXT-X-STREAM-INF:BANDWIDTH=400000,STEERING="1"
https://a.example.com/video/my-movie/high/index.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/high/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mfpy4iP$>

      "ID": "My-Backup-CDN",
      "TEMPLATES": {
        "1" : {
          "PREFIX": "https://b.example.com/backup-video<https://urldefense.com/v3/__https:/b.example.com/backup-video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44gKCaDMb$>"


If the player wishes to implement the switch to my My-Backup-CDN , it must actually match & replace (not prefix) the original URL, in order to convert

https://a.example.com/video/my-movie/mid/index.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/mid/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44hforWWE$>

in to

https://b.example.com/backup-video/my-movie/mid/index.m3u8

A prefix operation would give https://b.example.com/backup-videohttps://a.example.com/video/my-movie/mid/index.m3u8  which is clearly invalid.

However in the second relative mode, illustrated by

#EXTM3U
#EXT-X-CONTENT-STEERING:SERVER-URI="https://example.com/steering?video=00001<https://urldefense.com/v3/__https:/example.com/steering?video=00001&session=123__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qtYzaA4$>"
#EXT-X-STEERING-TEMPLATE:ID="1",PREFIX=""
#EXT-X-STREAM-INF:BANDWIDTH=200000,STEERING="1"
/mid/index.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=400000,STEERING="1"
/high/index.m3u8
This prefix operation is exactly what is required to convert

/mid/index.m3u8

in to

https://b.example.com/backup-video/my-movie/mid/index.m3u8

So we have an inconsistency in the algorithm that sits behind the use of “prefix” . This is never a good idea for players, which should rather perform the same operation consistently every time.

Proposal


  1.  Rename the master playlist #EXT-X-STEERING-TEMPLATE identifier PREFIX to be MATCH. This closer reflects its true functionality.
  2.  Implement this single algorithm in the player across all use cases:
     *   To create the NEW URL from the OLD URL given a master playlist MATCH value and a steering manifest  PREFIX value

  1.  Assign NEW to be the value of PREFETCH
  2.  Assign TEMP to be OLD URL
  3.  Delete from TEMP any part that matches MATCH. This operation is consistent even if MATCH is empty string.
  4.  Append TEMP to NEW to create the final URL.



Cheers
Will










From: Roger Pantos <rpantos=[e-mail]>
Date: Friday, February 26, 2021 at 9:42 AM
To: "[e-mail]" <[e-mail]>
Cc: http-live-streaming-review <[e-mail]>
Subject: Re: [Hls-interest] HLS Content Steering v1.0b1 preliminary specification

One objection we’ve heard to the approach below is that it requires the master playlist to be customized on a per-pathway basis. To address that we’re tweaking the proposal slightly: we're making the PATHWAY-ID attribute optional. If it is not present in the master playlist, the client will omit the _HLS_pathway query parameter from the steering manifest request until it obtains its first steering manifest.

(There are other ways for providers to communicate the initial pathway to the steering server. For instance, the SERVER-URI could be relative to the master playlist; each pathway could HTTP-redirect it to the actual steering server along with an origin identifier.)

Here are two examples of the current approach. The first one uses absolute URLs in the master playlist, as you might have with a dynamically-generated master playlist that knows the current pathway when the playlist is produced. The second uses relative URLs, as you might have for a static master playlist that is the same on all pathways.

In both cases the content is available here:

On the first CDN at -

https://a.example.com/video/my-movie/master.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/master.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qV1Gx_j$>

and on the second CDN at -

https://b.example.com/backup-video/my-movie/master.m3u8<https://urldefense.com/v3/__https:/b.example.com/backup-video/my-movie/master.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44llkyj51$>

-----

Absolute URIs (master playlist URI https://a.example.com/video/my-movie/master.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/master.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qV1Gx_j$>)

#EXTM3U
#EXT-X-CONTENT-STEERING:SERVER-URI="https://example.com/steering?video=00001<https://urldefense.com/v3/__https:/example.com/steering?video=00001&session=123__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qtYzaA4$>",PATHWAY-ID="My-Great-CDN"
#EXT-X-STEERING-TEMPLATE:ID="1",PREFIX="https://a.example.com/video<https://urldefense.com/v3/__https:/a.example.com/video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mqwdeF4$>"
#EXT-X-STREAM-INF:BANDWIDTH=200000,STEERING="1"
https://a.example.com/video/my-movie/mid/index.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/mid/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44hforWWE$>
#EXT-X-STREAM-INF:BANDWIDTH=400000,STEERING="1"
https://a.example.com/video/my-movie/high/index.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/high/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mfpy4iP$>

The client requests

https://example.com/steering?video=00001&_HLS_pathway=“My-Great-CDN<https://urldefense.com/v3/__https:/a.example.com/steering?video=00001&_HLS_pathway=**BMy-Great-CDN__;4oCc!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44h1w5BH2$>"

And gets back this steering manifest:

{
  "VERSION": 0,
  "TTL": 300,
  "RELOAD-URI": "https://example.com/steering?video=00001&session=123<https://urldefense.com/v3/__https:/example.com/steering?video=00001&session=123__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qtYzaA4$>",
  "PATHWAYS": [
    {
      "ID": "My-Great-CDN",
      "TEMPLATES": {
        "1" : {
          "PREFIX": "https://a.example.com/video<https://urldefense.com/v3/__https:/a.example.com/video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mqwdeF4$>"
        }
      }
    },
    {
      "ID": "My-Backup-CDN",
      "TEMPLATES": {
        "1" : {
          "PREFIX": "https://b.example.com/backup-video<https://urldefense.com/v3/__https:/b.example.com/backup-video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44gKCaDMb$>"
        }
      }
    }
  ]
]


-----

Relative URIs (master playlist URI https://a.example.com/video/my-movie/master.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/my-movie/master.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qV1Gx_j$>)

#EXTM3U
#EXT-X-CONTENT-STEERING:SERVER-URI="https://example.com/steering?video=00001<https://urldefense.com/v3/__https:/example.com/steering?video=00001&session=123__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qtYzaA4$>"
#EXT-X-STEERING-TEMPLATE:ID="1",PREFIX=""
#EXT-X-STREAM-INF:BANDWIDTH=200000,STEERING="1"
/mid/index.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=400000,STEERING="1"
/high/index.m3u8

In this case PREFIX=“” indicates that the steerable part is the entire (relative) URI. So after the client receives the steering manifest (same as above), if it has to switch to My-Backup-CDN it will just prepend its PREFIX to the steerable part of each URL to produce:

https://b.example.com/backup-video<https://urldefense.com/v3/__https:/b.example.com/backup-video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44gKCaDMb$>/mid/index.m3u8

https://b.example.com/backup-video<https://urldefense.com/v3/__https:/b.example.com/backup-video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44gKCaDMb$>/high/index.m3u8

Unless anybody spots any other major flaws in this approach, we’ll plan to update the preliminary spec and send out another version.


Roger.


On Feb 19, 2021, at 7:07 PM, Roger Pantos <[e-mail]<mailto:[e-mail]>> wrote:




On Feb 17, 2021, at 1:46 PM, Pieter-Jan Speelmans <[e-mail]<mailto:[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), ...

Well, we’re still early enough that we could consider changing the format. I’m not particularly moved by the hardship of supporting EXT-X-DEFINE — anybody who is implementing content steering is going to have to do at least that much work in their m3u8 parsing anyway — but I acknowledge that requiring content vendors to provide a different master playlists for old, un-updated clients is a non-trivial burden.

What if we took the following approach instead?

First, add a new STEERING attribute to regular EXT-X-STREAM and MEDIA tags:

#EXT-X-STREAM-INF:BANDWIDTH=200000,STEERING=“1”
https://a.example.com/video/mid/index.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/mid/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44myYNLnQ$>

#EXT-X-STREAM-INF:BANDWIDTH=400000,STEERING=“1”
https://a.example.com/video/high/index.m3u8<https://urldefense.com/v3/__https:/a.example.com/video/high/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44j6ekA61$>

This links the variants (and renditions) to a steering template definition:

#EXT-X-STEERING-TEMPLATE:ID=“1”,PREFIX="https://a.example.com/video<https://urldefense.com/v3/__https:/a.example.com/video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mqwdeF4$>”

#EXT-X-CONTENT-STEERING:SERVER-URI="/steering?video=00001",PATHWAY-ID="My-Great-CDN"

The manifest would change a bit:
{
   "VERSION": 0
   "TTL": 300,
   "RELOAD-URI": "https://example.com/steering?video=00001&session=123<https://urldefense.com/v3/__https:/example.com/steering?video=00001&session=123__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44qtYzaA4$>",
   "PATHWAYS": [
       {
"ID": "My-Great-CDN",
"TEMPLATES": {
  "1" : {
“PREFIX”: "https://a.example.com/video<https://urldefense.com/v3/__https:/a.example.com/video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mqwdeF4$>"
  }
}
       },
       {
"ID": "My-Backup-CDN",
"TEMPLATES": {
  "1" : {
“PREFIX”: "https://b.example.com/backup-video<https://urldefense.com/v3/__https:/b.example.com/backup-video__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44gKCaDMb$>"
  }
}
       }
   ]
}

The client would load and play the master playlist as usual. When it parsed the EXT-X-STREAM-INF and EXT-X-MEDIA tags it would see the STEERING attribute and verify that there is a corresponding EXT-X-STEERING-TEMPLATE and that the PREFIX of the steering template matches the start of the URI (fatal error if either check fails). It would store away the remainder of the URI as the “steerable part.”

When it loaded the manifest it would check that each pathway had an object in its TEMPLATES object for each STEERING-TEMPLATE (matching the key to the ID). (If it did not, steering would be disabled.)

To switch pathways it would concatenate the PREFIX from the TEMPLATES object with the steerable part of each URI. After that, everything would behave the same way.

This approach isn’t as flexible as general variable substitution. But I think that it would still cover the majority of the substitution use cases (and I’d be interested to hear about those that it doesn’t).


Roger.



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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