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

[Roger Pantos <[e-mail]>]

Hello Will, response inline:

> On Feb 26, 2021, at 3:38 PM, Law, Will <wilaw=[e-mail]> wrote:
> 
> Another question on this new proposal: can the spec clarify whether the client is required to request the steering manifest and resolve the pathways beforeit loads any media playlists? Given the playlist below
>  
> #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$>
> #EXT-X-STREAM-INF:BANDWIDTH=400000,STEERING="1"
> https://a.example.com/video/my-movie/high/index.m3u8 <https://urldefense.com/v3/__https:/a.example.com/video/my-movie/high/index.m3u8__;!!GjvTz_vk!E3AorzSNM8BjAiM9Qaz2_XmVx2NyOl_J1ZzrkyQCOBZuw0Khr__44mfpy4iP$>
>  
> a client could start by immediately loading https://a.example.com/video/my-movie/mid/index.m3u8 <https://a.example.com/video/my-movie/mid/index.m3u8>, and then resolving the steering manifest in the background after it had built up a few seconds of playback buffer. Players would be tempted to do this as it would provide a lower VST (by one RTT and whatever processing time there is on the steering server). This would be a bad outcome for two reasons a) For QoE as there could potentially be two cold hits against CDNs in the first 10s of playback   - one for the default and the second as the new pathway kicks in and b) if you are trying to move away from an underperforming default pathway your startup sequence would continue to be pinned against it.
>  
> The current version of the steering draft is ambiguous. It says “When the player needs to poll for Pathway updates,", but does not dictate when this should happen with respect to conventional start-up.
>  
> Suggestion is that if the player understands steering, it MUST resolve the steering manifest prior to loading any media playlist.

We tried to make this clear in the Player Behavior section:

1. When playing a Master Playlist with an EXT-X-CONTENT-STEERING tag, load the Steering Manifest in parallel with the media playlist and segments requests, composed using the default values of the EXT-X-DEFINE tags.

In other words what we expect is that the player will begin loading the media using the steering path expressed directly in the master playlist. As Daniel mentioned in his reply, to do otherwise would insert an extra round trip into the startup phase of playback.

We expect that the dominant case will be that whoever hands out the master playlist to the player — be it dynamically generated, or a static master playlist with relative media playlists selected on a particular CDN — will do so in agreement with its steering server, providing a master playlist that describes the currently-preferred pathway. In the case where it does not, the steering response will race with the media playlist / segment requests and cause an immediate pathway reselection. There’s a bit of inefficiency there due to the extra round trip and canceled requests, so I would encourage providers to take care to avoid hitting that case.

Note that having _HLS_pathway or some other indication of master playlist origin in the steering manifest request can assist the steering server in keeping the client on its current pathway.


Roger.

>  
> -Will
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> From: Roger Pantos <rpantos=[e-mail] <mailto:rpantos=[e-mail]>>
> Date: Friday, February 26, 2021 at 9:42 AM
> To: "[e-mail] <mailto:[e-mail]>" <[e-mail] <mailto:[e-mail]>>
> Cc: http-live-streaming-review <[e-mail] <mailto:[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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