Implement clean track switching

[tobbee]

Implement clean switching between tracks in the same render group like video or audio tracks.

Use Absolute Joining Fetch (code 0x03) as part of the following scheme recommended by Gwendal Simon and Will Law:

1. Client is playing Group N of track A and wishes to switch to track B
2. SUBSCRIBE B (low-priority)
3. ABSOLUTE_JOINING_FETCH B with Joining_start = X (high priority)
4. SUBSCRIBE_UPDATE A end=X
5. Wait for the FETCH to conclude (or get close to conclude)
6. SUBSCRIBE_UPDATE B (high priority)

The client is the one who decides the target value X for switching. X should be greater than N and depends on

• the urgency to switch

  • for example if congested, the client can set X = N+1 to make it asap, or even X=N for an emergence switch down
  • for example if clients plays an object close to the end of the group N it may choose X=N+2 instead of N+1
  • for example if it is an up-switch, it may not matter to force a switch too early in the future at the risk of getting redundant A and B objects

• the knowledge about the future group ID and the timing

  • for example, if groups are very short or very long RTT client-server, the client can target the group that
    corresponds to the time it takes to get the first FETCH object
  • for example, if gaps in the group ID, the client get the info of the next group/timing by another mean (timeline track?)
  • for example if it switches because it knows that, starting at group X, the content will be differen

• the current live object of track B, which is obtained by the SUBSCRIBE_OK after step 2

  • for example the ABSOLUTE_JOINING FETCH can be omitted if the current live group of B is N+1.

Slides describing describing this and other alternatives are available
here.

[tobbee]

A first approach will rather be:

sequenceDiagram
    participant Subscriber
    participant Publisher
    participant TrackA
    participant TrackB

    Subscriber->>Publisher: 1. SUBSCRIBE A (priority 128)
    Publisher->>TrackA: Activate Track A
    
    Note over Subscriber, TrackB: Initial state: Track A is active

    TrackA-->>Subscriber: Media objects (groups ...N)

    Note over Subscriber, TrackB: Switch initiation

    Subscriber->>Publisher: 2. SUBSCRIBE B (next_group, priority 127)
    Publisher->>TrackB: Activate Track B
    Publisher-->>Subscriber: SUBSCRIBE_OK B (largest_location: group_N, object_X)
    
    Note over Subscriber, TrackB: Parallel delivery period (group N+1)
    
    par Track A continues
        TrackA-->>Subscriber: Group N+1 objects (priority 128)
    and Track B starts
        TrackB-->>Subscriber: Group N+1 objects (priority 127)
    end

    Note over Subscriber, TrackB: End Track A delivery

    Subscriber->>Publisher: 4. SUBSCRIBE_UPDATE A (end_group N+1)
    Publisher->>TrackA: Deactivate Track A
    
    Note over TrackA: Track A ended (may have sent some N+1 objects)

    Note over Subscriber, TrackB: Restore priority for future switches

    Subscriber->>Publisher: 6. SUBSCRIBE_UPDATE B (priority 128)
    TrackB-->>Subscriber: Continue with group N+2... (priority 128)

    Note over Subscriber, TrackB: Track B is now the active track (priority 128)

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