Shaka 2.0 and stream interruption

[agelun]

Latest master (6d0f081)

According to http://dashif.org/wp-content/uploads/2015/04/DASH-IF-IOP-v3.0.pdf "4.8.3" and "4.8.5" (page 67) we have two options to indicate temporary stream unavailability:

1. "Short Periods" – "This expresses that for this Period no media is present at least for the time as expressed by the @duration attribute". As I understood, they mean periods without any AdaptationSets or Representations inside.
2. Using "dummy content", and according to "4.8.5" sending 404 to indicate content unavailability for specified interval.

But in first case Shaka reports about invalid MPD (no AdaptationSet/Representation) or stucks before such "dummy" period (in case of stream info absence).

In second case (404 response) Shaka makes repeated attempts to get a chunk and doesn't continue playback until we move scrollbar (or until chunk suddenly appears).

What do you recommend to use for temporary stream unavailability indication?

Are you going to handle 404 responses and "dummy periods" according to DASH-IF-IOP?

Thank you.

[agelun]

And the simpliest way is to support discontinuity between periods (when period1.start + period1.duration < period2.start). I've already made a patch for this and it would be great if you'll be able to review it.

[joeyparrish]

We don't currently support discontinuity at all, but I'd love to see your patch. Perhaps I have been over-complicating it.

As for short periods and dummy content, we don't have any such content available. Can you provide a test stream so that we can take a look?

[agelun]

Thanks, Joey. you can find my commit here:
https://github.com/agelun/shaka-player/commit/d0b3fe58bf58f8ca72f904b37e199db104513402

I didn't make a pull request because it breaks a lot of test cases and it's not ready for merge.

[joeyparrish]

@baconz, this seems related to a conversation we were having about gaps.

@agelun, it appears that you detect when the playhead is in a gap, and move it to the other side. This is something we have considered before and decided not to do.

I took a look at the sections of the IOP you referenced. (The same content appears in the same sections in the most up-to-date version of IOP, v3.3.)

My reading of the IOP is that there are three mechanisms:

1. Dummy content. This means that the server generates or uses pre-made media to fill the gap in the input. The output DASH stream has no gaps. Dummy content requires no player support, so this is a definite option for you. (Note that in my interpretation, this is not the same as serving a 404.)
2. "Short Periods". These are periods with no media at all. We do not support these currently.
3. 404s. The Period element points to media which is not there on the server. When the segment is requested, we get a 404 and we know that there is no such segment. We do not support this either.

It appears that using 404s is the same as short periods in concept. Both indicate that segments are not available and will not be available. The main difference is in when the player finds out. A short period tells the player at manifest parse time that there's no content. A 404 when fetching a segment tells the player later, when it starts trying to buffer the relevant section of the presentation.

I dislike using 404s for this because there are other ways to encounter them, such as bad clock sync or a misbehaving server. If the player is expected to happily move past 404s, it may make debugging certain server problems more difficult.

Short periods are more clear, but still don't fit into our current architecture. We don't expect to be able to play through gaps, and these empty periods just signal "here's a gap".

I'm not sure if/when we would be able to support gaps. Because of the way MediaSource works, I believe it is difficult for JS-based players to deal with gaps elegantly. I'm averse to extra complexity in the player.

[baconz]

@joeyparrish there's one more option that we discussed, which would be to leave out the empty period, but apply an adjustment to the live head, pulling it back by a duration equivalent to the gap length. This method is not yet supported in the spec, but I think that barring a true server-side solution (ie filling the gaps with a "slate"), it might be the simplest fix.

I'm happy to submit a ticket to the IOP if others think that this would be helpful.

[sandersaares]

I would like to point out some additional complexity, especially with live streams. Empty periods and dummy content might be technically feasible for on-demand content but for live content, the backend often might not even have the ability to detect missing pieces, let alone do something about them. Many packagers simply deliver segments over FTP to a CDN origin and if one segment gets lost then too bad - it will just skip the segment and carry on.

This should not happen but real world network conditions can be nonideal, so I have seen it quite a lot in practice - there's a gap of one or more segments, after which the stream continues just fine. Maybe not very often but perhaps just once a day. If players do not skip past 404s, this means that live streams will suddenly stall at random moments in time, for all (or a large subset of) users simultaneously. This makes users rather unhappy.

[agelun]

@joeyparrish can you explain why did you decide not to "jump over" the gap? Are there any tech reasons or just code style/simplicity/etc.?

I thought about #462 as well and both cases can be solved if you create something like "event bus" for stream events, I think. There could be pushed MPD events from EventStream tag and discontinuity events (in case of periods interruption, empty periods and 404 responses – all from different sources) and application will be able to choose when/how to react on such events (for example, jump over, stuck, show error message etc. for discontinuity events). It can be used to draw informative timeline as well.

I need to understand how can we progress with this issue, should we support our own branch to cover such "real life" requirements or it can be merged into google's code. Or, maybe, there are some more serious reasons I don't know about and it cannot work well at all?

[joeyparrish]

We decided not to jump over the gap for many reasons, but I think they are becoming increasingly irrelevant. Let's figure out the best solution and work toward getting it merged into Shaka Player.

In my view, there are many reasons for gaps:

1. Broken content. The fact is that we get many support requests that turn out to be bugs in the encoder or bad input to the encoder. See DASH stream playing on DASH IF but not in JW Player #562. Broken content and non-IOP-conforming content are things we don't intend to support. I think failing on broken content is appropriate and helps to highlight and fix bugs in encoders. Jumping gaps here may hide these content issues, but that shouldn't be a deal-breaker.
2. Browser bugs. As of this writing, Chrome is still mixing up DTS and PTS, which can sometimes cause gaps to appear at the start or end of a Period even when there are none in the content. Jumping gaps in this case might be a good thing, in that it would work around browser bugs when possible.
3. True discontinuity. This is permitted by IOP and a reality of live streaming for encoders which don't generate dummy content. Jumping gaps in this case is a clear win, as it allows us to play live streams we couldn't otherwise play.

I don't see any good way to differentiate between these three cases, so probably it's best to try to handle all gaps.

One thing that worries me is the use of 404s to signal discontinuities. There are other times a 404 may occur:

1. Again, broken content. If I mis-encode VOD content, segments might be missing or their URLs might be wrong in the manifest. Ideally, we would fail on such content so that the app developer sees an error and can start investigating.
2. Live content when clock sync is broken or misconfigured. If the app config is wrong, the player might request past the proper live edge and get a 404 from the server. In this case, treating a 404 as a discontinuity would result in the player requesting the next segment after that, and so on. The rest of the stream would appear as one big gap and would generate no errors. Ideally, we would fail with an error in this case, too.
3. True discontinuity. These could appear at the live edge or elsewhere, depending on whether a time shift buffer is used. Ideally, we should handle this gracefully somehow.

I'm unsure how to differentiate the third case from the other two. Perhaps we could have a heuristic for that, such as isLive && numConsecutive404s < errorThreshold. If there are too many in a row, we would decide we were in case 2 and fail. If it was VOD, we would decide we were in case 1 and fail.

We've also discussed two different strategies for handling gaps:

1. Seek over gaps automatically. I'm concerned about the complexity of this, but it could work. Playhead seems the most reasonable place to detect and jump gaps. It could be similar to how we keep the playhead inside the seek range.
2. Dispatch events to allow the application to decide what to do. If the app isn't expecting gaps, it could fail. This offers greater flexibility, but requires more effort on the part of the app developer. Extra work is required for any app expecting gaps, to listen for that and jump over them. Extra work is also required for any app not expecting gaps, to listen for them and fail.

Thoughts? Specifically on:

• decision not to differentiate causes of gaps in JavaScript
• heuristic to differentiate between good 404s and bad ones
• pros/cons of strategies for jumping gaps

[agelun]

Hello, Joey. Thanks for reply.

Step by step:

1. Broken content. Yes, It's a encoders and/or IP delivery (in case of multicast delivery) problem. But end user don't want to know it )) So it's better to "hide" such problem sometime instead of shout "Hey, we/our streaming partner have dumb network admin, he cant configure QoS" )
2. 404 with VoD content. I can agree with you in this part. Usually content provider is able to check content consistency before publishing. Especially in case of "SegmentBase" profile.
3. 404 with Live. Maybe it'll be better to check server time from HTTP response header (?)
4. Handling strategies. I think only second one will be "production friendly" because:

• Automatic seek over gap can be surprising for end-user. Usually app developer wants to notify somehow about problem, switch to "stub" advertisement or disable such seek at all (for example when you'll be switched to another film. Imagine, your kid watched cartoons and suddenly was switched to some horror blockbuster )) )
• Generated event can be used for monitoring purposes as well (to notify monitoring system about such discontinuity/gap)
• Discontinuity events can be unified with other stream events (e.x. EventStream/Event from MPD). So it'll be more generic and more elegant.

And I don't understand why you think it'll be more complex for app developer. Event can be ignored and playback will stop (like now).