Re: [Moq] Flow to join 15 seconds before live

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

I have also taken a stab at defining the requirements:

R1 – as a receiving client, I want to be able to join a track, starting at the latest available group, without prior knowledge of the latest group number.
R2 – as a receiving client, I want to be able to join at track at an explicit start group and then receive all future groups from that point on.
R3 – as a receiving client, I want to be able to fetch a discreet range of groups, from a start group to an end group that I specify.
R4 - as a receiving client, I want to be able to discover what is the latest available group in a track without having to subscribe to that track.
R5 - as a receiving client, I want to be able to define a relative priority between my active subscriptions and fetches.
R6 - as a receiving client, I want to be able to define whether data in a subscription or fetch is delivered with Ascending or descending Group number.
R7 – as a receiving client, I want to be able to instruct the sender to drop lower groups in favor of higher number groups.

I have doubts that we can enable R6 with the current permissive definition of ascending-but-not-consecutive group numbers and I include this requirement to respect Suhas’prior requirements. My concern is this:  if a relay has received group 16, 18, and now it receives group 22, can it forward it? It has no idea that group 20 is delayed. The reality is that with a live stream, the relay NEVER knows if there are out-of-order groups. The same is true for non-cached VOD. A relay knows what is has in cache, and can order those, however if it doesn’t have the requested groups in cache and has to request them from upstream, that again it does not know if there are gaps and therefore cannot deliver a reliable order. I feel R6 is a red herring. Instead, relays should always forward groups in the order they receive them and this will usually be ASC by nature of our definition of group number. This is decoupled from the congestion-response behavior of dropping lower groups in favor of higher groups . This requirement is expressed as R7 and it can always be done by the relay if requested by the client.

Cheers
Will


From: Moq <[e-mail]> on behalf of Alan Frindell <afrind=[e-mail]>
Date: Monday, March 25, 2024 at 10:25 PM
To: Suhas Nandakumar <[e-mail]>, Ted Hardie <[e-mail]>
Cc: "Law, Will" <wilaw=[e-mail]>, MOQ Mailing List <[e-mail]>
Subject: Re: [Moq] Flow to join 15 seconds before live

Hi Suhas, thanks for up leveling the discussion to requirements.  My recommendation is that we focus on getting agreement on those first, before moving to identify which solutions address which ones to what degree.

--
Req-01: As a receiving client, I want to be able to find out if there are any gaps (one or more of them) in the objects being published for the thing I asked for.

Req-02: As a receiving client, I want to have data delivered to be in the oldest to newest order of the groups to support my playout buffer.

Req-03: As a receiving client, I want to have fed the data as it is produced and I don't know absolute start or end locations.

Req-04: As a sending client, I want to mark what data is important for my application.
--

You used slightly different language for Req-03, I interpret it to mean “As a receiving client, I want to join the track at the live head without knowing the absolute start or end locations” – is that correct, or is there more you are trying to express?
What do other folks think about the requirement list – any additions or amendments?

Thanks

-Alan


From: Moq <[e-mail]> on behalf of Suhas Nandakumar <[e-mail]>
Date: Monday, March 25, 2024 at 1:38 PM
To: Ted Hardie <[e-mail]>
Cc: "Law, Will" <wilaw=[e-mail]>, MOQ Mailing List <[e-mail]>
Subject: Re: [Moq] Flow to join 15 seconds before live

Thanks Cullen for starting this thread. Overall I like the simplicity of the proposal. After reading the commentary, I do think we need to take a step back and establish some set of requirements . I will lay out my understanding of the requirements
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Thanks Cullen for starting this thread. Overall I like the simplicity of the proposal.  After reading the commentary, I do think we need to take a step back and establish some set of requirements . I will lay out my understanding of the requirements at the MOQ Transport layer to support various applications ( vod, live streaming, interactive, ad insertion, gaming, side channel catchup, chat and so on).


Req-01: As a receiving client, I want to be able to find out if there are any gaps (one or more of them) in the objects being published for the thing I asked for.

Req-02: As a receiving client, I want to have data delivered to be in the oldest to newest order of the groups to support my playout buffer.

Req-03: As a receiving client, I want to have fed the data as it is produced and I don't know absolute start or end locations.

Req-04: As a sending client, I want to mark what data is important for my application.

Let me take a stab at few API semantics being proposed against these requirements

(1) Subscribe with no locations but with a clear list of options

Req-03 can be done.
Req-02 conflicts with Req-03. Also fails when it comes to unambiguously identifying Relay behavior and limits de-duping subscriptions under conflicting requests. ( Please note we have the same problems when subscribing with relative locations as defined in draft-03)
Req-01 can be probably done, however it conflicts with Req-03 since finding out gaps might need searching through relay hierarchy and/or eventually going back to the original publisher.

(2) Fetch with only absolute locations

Req-01 can be done
Req-02 can be done
Req-03 conflicts with Req-02.

(3) Subscribe with Delivery Order for Groups (oldest to newest / newest to oldest), not Fetch-like operation

Req-01 and Req-03 conflict
Req-02 can be done, however defining any RECOMMENDATION for the relay behavior will be inappropriate under conflicting requests for the delivery order. This causes receiving clients for the same application to have different experience based on the delivery order picked by a given relay ( or set of Relays)

Next let's look at the requirements on group delivery order preference ( oldest group first or newest group first) and who controls it (original sender or every receiver)

(1) Every receiver (end receiver, relays) of a track can ask  a given delivery order preference for a group. This is the most flexible approach and seems powerful at first. But when we start building relays and have to manage conflicting requests ,things get challenging ( as described above)

(2) Sender sets the group delivery preference. Since the sender controls the application  data, it  sets the needed group delivery order preference for a given track. Relays use the group delivery order preference and object priority to decide the ordering of groups and objects within a group when making delivery decisions. Note, this doesn't remove the need to move API semantics that solves Req-01, however.

Fetch in itself can be "cleanly" applied to several use-cases that cannot be easily done with Subscribe without causing semantic ambiguities ( partial data at relay, finding gaps, who to ask for the data)


Lastly, on Freeze for Subscriptions
 The option to freeze a subscription request is still needed outside the use-cases of fetch  vs subscribe. It helps pre-warming the relay cache, it helps moving between qualities for ABR to be realized within a single hop length  ( avoiding the need to go back deeper into the Relay network), for example.

I feel the following should meet most of the use-cases MOQ can eventually help address
- Subscribe with modes ( prev, now and next)
- Fetch with absolute ranges
- Sender setting group delivery order preference
- Sender setting object priorities

Cheers
Suhas


















On Mon, Mar 25, 2024 at 4:21 AM Ted Hardie <[e-mail]<mailto:[e-mail]>> wrote:
Just so I understand the use case, is the choice of 15 seconds because of a control presumed to be on the player (a "go back N seconds"/"go forward N seconds" control)?

I ask this because the ease of delivering this functionality depends a lot on whether the catalog producer has provided a mapping.  For a catalog with no mapping between groups and timings, this pretty much has to be done via a heuristic; for one with a well-described mapping, it should be trivial to extract.  If that is the case, I think it might make sense to say that the MoQT protocol provides the ability to request group ids/object ids earlier than live edge, but that mapping those to timing is not a function of the transport.   Any publisher that wants to provide that functionality has to do so in the catalog or with a timing track (and in a lot of cases, it will know whether or not the app has that type of control).

If that were the case, then the only thing MoQT needs to do is to tell you the group number/object number of the live edge (as seen by the relay you're on).  The mechanics from that point are client driven.

From my perspective, the next question is then whether you expect the client to have a playout buffer that allows it to handle re-ordering up to the 15 second mark.  If that is the case, then the subscribe with a group number/group id target makes sense to me, because the cache can start delivering whatever it has (including the live edge) and fill in the buffer as it gets older data, knowing that it is up to the client to hold that and start playback when it has the data it wants to send to the decoder.

If you don't expect the playout buffer to be that large, then I think you do need to get the group/objects in order.  That requires either the relay to buffer until it can do that or something like what Cullen put forward, since that allows the client to drive the process in a way that gets the data in a specific order without going all the way to using fetch for the parts after the live edge.

Just my perspective,

Ted



On Mon, Mar 25, 2024 at 10:03 AM Law, Will <wilaw=[e-mail]<mailto:[e-mail]>> wrote:
Thanks Cullen for initiating this thread and Cullen and Luke for proposing solutions.

I find the proposed worklow to start N seconds behind to be unnecessarily convoluted, As proposed, it requires the player to make SUBSCRIBE request which is “frozen” , followed by FETCH request, followed by a SUBSCRIBE “unfreeze”. In comparison, our workflow for joining a real-time track is clean – a single SUBSCRIBE and all received objects can be piped to a decode buffer and rendered. We should aspire to an equally clean workflow for non-real-time and VOD playback. These are our two majority join use-cases.

I am not of fan of frozen=true. This is overloading SUBSCRIBE to return state information about the track. This would be better off being moved to an explicit control message, perhaps (as Luke just proposed) TRACK_INFO.

I also support the suggestion to remove relative start and end groups from SUSBCRIBE. This is causing more problems than it is solving. Real-time players don’t care about relative offsets because they always want the latest group. Non-real time players have the luxury of time . The key difference with starting N seconds behind live is that we can afford to spend one RTT discovering things (assuming RTT << N). So a clean workflow would be:


  1.  Client makes the MOQT connection.
  2.  Subscribe to the catalog. The catalog tells it the track names and also details of their numbering schemes and group duration.
  3.  For the track it wants to starts with, request TRACKINFO(track) (or a timeline track, if this is offered by the streaming format).
  4.  If the relay has an active subscription, it can immediately return the latest group number. Otherwise it makes the same request upstream and then relays the answer to the client. The client now knows the latest group number.
  5.  The player uses its knowledge of the latest group, the numbering scheme, the group duration and the desired starting buffer to calculate the starting group G.
  6.  It then makes a single FETCH(startgroup=G endgroup=undefined priority=X). The response from this can be piped directly into its decode buffer. (note, this is a changed API from what we currently have)

We have two options for step 6.

  1.  We can extend SUBSCRIBE to allow a reliable ASCending delivery mode, or
  2.  We can extend FETCH to allow an undefined end group i.e keep delivering into the future until end-of-track is signaled (as I showed above).

I don’t have religion on either of these options, although separating the reliable and ordered nature of FETCHed delivery into its own API seems reasonable and preferable.

Cheers
Will


From: Moq <[e-mail]<mailto:[e-mail]>> on behalf of Luke Curley <[e-mail]<mailto:[e-mail]>>
Date: Monday, March 25, 2024 at 6:58 AM
To: Cullen Jennings <[e-mail]<mailto:[e-mail]>>
Cc: MOQ Mailing List <[e-mail]<mailto:[e-mail]>>
Subject: Re: [Moq] Flow to join 15 seconds before live

Hey Cullen, thank you very much for writing down the proposed flow.

The problem with this approach is that while there is head-of-line blocking on startup (via FETCH), there is no head-of-line blocking during the steady state (via SUBSCRIBE). Any congestion will cause the player buffer to shrink, and unfortunately it will be filled in reverse group order. This makes sense for low latency playback as it enables skipping content, but it will cause excessive buffering for high latency playback that does not skip content.

The other big issue in #421<https://urldefense.com/v3/__https:/github.com/moq-wg/moq-transport/pull/421__;!!GjvTz_vk!WGTeZi5pgPzPD8mxX8VyAOgHY3oe8Z_BFDyrNfOXtsABW1UxHKoIHOqmfXHD1ou-vdwSAVggUnzwWeB-DpLBTseeQjsm$> is that we still need the ability to SUBSCRIBE at absolute group IDs, otherwise ABR won't work because of numerous race conditions. Even a back-to-back UNSUBSCRIBE 720p + SUBSCRIBE 240p are evaluated at different times based on different cache states (including potentially empty). I can't think of any algorithm that works using only relative SUBSCRIBEs, especially if groups are not aligned between tracks.


As I see it, the main problem with SUBSCRIBE today is the RelativeNext and RelativePrev start/end ID. These are both hard to reason about and implement; what if we removed them altogether? I propose that a SUBSCRIBE could only start/end at an absolute ID or the latest group.

And then we add SUBSCRIBE order=ASC|DESC (#411<https://urldefense.com/v3/__https:/github.com/moq-wg/moq-transport/issues/411__;!!GjvTz_vk!WGTeZi5pgPzPD8mxX8VyAOgHY3oe8Z_BFDyrNfOXtsABW1UxHKoIHOqmfXHD1ou-vdwSAVggUnzwWeB-DpLBTu3MfrWi$>) to indicate if head-of-line blocking is desired, supporting both VOD (old content) and reliable live (new content). I do think we also need SUBSCRIBE priority=N (also #411<https://urldefense.com/v3/__https:/github.com/moq-wg/moq-transport/issues/411__;!!GjvTz_vk!WGTeZi5pgPzPD8mxX8VyAOgHY3oe8Z_BFDyrNfOXtsABW1UxHKoIHOqmfXHD1ou-vdwSAVggUnzwWeB-DpLBTu3MfrWi$>), as otherwise it's undefined how multiple subscriptions/fetches interact once we can conditionally ignore send_order, but that's a bigger discussion.


Reliable Live (Fixed Groups)
-> SUBSCRIBE    range=live..   order=ASC priority=1
<- SUBSCRIBE_OK range=823..
-> SUBSCRIBE    range=818..823 order=ASC priority=0
<- SUBSCRIBE_OK range=818..823

Optional: The first subscribe is lower priority than the follow-up, allowing it to both learn the latest group (823) and use the first RTT to warm the cache/connection with eventually useful data. Your proposed frozen=true is okay but conceptually it's behaving as a HEAD request, which I think we should add anyway (TRACK_INFO?).

Reliable Live (Dynamic Groups)
-> SUBSCRIBE    track=timeline range=live.. order=DESC
<- SUBSCRIBE_OK track=timeline range=823..
-> SUBSCRIBE    track=video    range=818..  order=ASC
<- SUBSCRIBE_OK track=video    range=818..

Optional: The timeline track has aligned groups in this example but you should parse the received OBJECTs instead. It's still going to be 1RTT and the timeline track needs to be fetched anyway for DVR support.

On Sun, Mar 24, 2024, 3:15 PM Cullen Jennings <[e-mail]<mailto:[e-mail]>> wrote:

At the IETF meeting I said I would write something up about how a MoQ client could join and get the video for 15 seconds behind the live edge.


The goal here as I understand it is let the the client get say 15 seconds of video before the the current live edge then start playing roughly that far behind. The reason it would be 15 seconds behind would be e just to create a play out buffer and be able to do things like shift to a lower bitrate subscription (client side ABR style)  if the network was getting crappy without having a stall in the play out. This is not a use case I do so I might be missing something in this but I do want to make sure this use case works if it is important for other people.


Here is how I am thinking about client could do this:

Step 1:

Discover a relay and setup the TLS/ QUIC / MoQ connection to it.

Step 2:

Subscribe to the catalog and get the catalog information. From this learn the track name but also learn that each group is 5 seconds long ( so 3 groups for 15 seconds ). For things that use variable groups sizes, subscribe to the track that gives the mapping from time to group numbers and get the latest data from that.

Step 3:

Subscribe to Track with start=next, and frozen=true.

This will cause the relay to get the information from the upstream if it does not already have it and return information about the live head. Note that if the relay already has a subscription for this track, it does not need to do anything, just retun OK with this relays view of the current live edge.

Relay will return a Subscribe OK with the live edge object - for example, lets say it is group=1234 object=5,

Step 4:

At this point the client knows it needs to go 3 groups back from 1234 so it needs groups 1231 to 1234.

Client sends a Fetch of Group 1231 to 1234 with the direction set to normal not reverse. If the relay. is missing some of this in the relays cache it will request it upstream. The relay will send an OK and start sending all the objects in those three groups. Note that if the relay got several clients joining at same time, and the first requested 1231-1234 and the second client requested 1230-1233, the relay MAY do the optimization book keeping to send then  1231-1234 upstream then for the second just send 1230-1230 upstream as it already has requested the others.

The client will receive all the objects for the three groups in the order of the group id / object id.

Step 5:

When the last object in group 1234 arrives, the client sends and subscribe Update that changes the freeze to false in the original subscribe and causes objects from the subscription to start going to the client.

( and yes we need a way to know what the last object in a group is but that is a separate issue. We agree we will have some way of doing this even though we are not sure exactly what that way is yet )

At this point the client will start receiving objects from group 1235 and future groups.


>From a processing point of view, the client does pretty much the same thing when it gets an object regardless of if it came from Fetch or Subscribe.

Few questions on this:

A. Before we get into if this is the optimal solution for this, am I understand the problem and use case correctly ?

B. Does this explanation make sense and does this solution work ?

C. What is uggly or unfortunate about this solution and how big a deal is that ?






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