Fix bandwidth probation dead state

[vpalmisano]

Running some tests with high packet loss (~20%), in most cases the server stops sending any video packet and we never exit from that state, even if the network link improves.

Looking at the ALR detector (that should be activated when there are no RTP packets sent over the transport), it seems that it is falling into a dead state:

1. in NetworkControlUpdate GoogCcNetworkController::OnProcessInterval we periodically call start_time_ms = alr_detector_->GetApplicationLimitedRegionStartTime() setting the output value to probe_controller_->SetAlrStartTimeMs(start_time_ms)
2. the AlrDetector state is set by AlrDetector::OnBytesSent, so when the transport stops sending packets, we could end up in this state:

} else if (alr_budget_.budget_ratio() < stop_budget_level_ratio_ &&
           alr_started_time_ms_) {
  state_changed = true;
  alr_started_time_ms_.reset();
}

3. at this point GoogCcNetworkController::OnProcessInterval calls probe_controller_->SetAlrStartTimeMs(<empty value>) with this empty value set into the ProbeController, and the ProbeController::Process never calls InitiateProbing, so no more probing is created;
4. the bandwidth estimation is never tested again, so we never resume sending packets.

With this PR we use a alr_timeout_ value (set to 3s by default). After that timeout, the ALR resumes sending probes.

[ibc]

Is this something available in latest libwebrtc version? You are basically changing libwebrtc. Also, we have an ongoing PR #922 upgrading to latest libwebrtc so I don't see a point in doing it in current very old version.

[ibc]

@sarumjanuch you may want to see this.

[vpalmisano]

Is this something available in latest libwebrtc version?

Yes, it seems the same of the latest libwebrtc code: https://webrtc.googlesource.com/src/+/refs/heads/main/modules/congestion_controller/goog_cc/alr_detector.cc

[sarumjanuch]

I am aware of this. In my branch it is handled. This is from 18.11.2022:

[ibc]

Ok, let me take a proper look on Monday and merge it in v3. But it will generate conflicts in your branch @sarumjanuch, is it ok?

[sarumjanuch]

Don't worry at all. My branch is already deviated a lot, both, from ms and libwebrtc.