BWE increases slowly when audio packets are sent before video packets.

[giangndm]

This PR have some changes:

• Fix some missing when porting bitrate_estimator.cc from libwebrtc.
• Added bwe reset API

For more clearly, I will explain what issues lead to this PR.

Problem

When an audio stream starts before a video stream, the Bitrate Estimator (BWE) calculates the initial bitrate based on the audio packets. This results in a very low estimated bitrate, typically around 40kbps. When the video stream starts, the BWE struggles to adapt to the increased bandwidth requirements, leading to a slow increase in bitrate. This can cause poor video quality.

        let sample_estimate_bps = sample_estimate.as_f64();
        let estimate_bps = estimate.as_f64();
        // Define the sample uncertainty as a function of how far away it is from the
        // current estimate. With low values of uncertainty_symmetry_cap_ we add more
        // uncertainty to increases than to decreases. For higher values we approach
        // symmetry.
        let sample_uncertainty =
            scale * (estimate_bps - sample_estimate_bps).abs() / (estimate_bps.max(25_000.0));
        let sample_var = sample_uncertainty.powf(2.0);

        // Update a bayesian estimate of the rate, weighting it lower if the sample
        // uncertainty is large.
        // The bitrate estimate uncertainty is increased with each update to model
        // that the bitrate changes over time.
        let pred_bitrate_estimate_var = self.estimate_var + 5.0;
        let mut new_estimate = (sample_var * estimate_bps
            + pred_bitrate_estimate_var * sample_estimate_bps)
            / (sample_var + pred_bitrate_estimate_var);
        new_estimate = new_estimate.max(ESTIMATE_FLOOR.as_f64());
        self.estimate = Some(Bitrate::bps(new_estimate.ceil() as u64));
        self.estimate_var =
            (sample_var * pred_bitrate_estimate_var) / (sample_var + pred_bitrate_estimate_var);

https://github.com/algesten/str0m/blob/a3e0f13744ef8b0681bd34b97298dbe72085878a/src/packet/bwe/acked_bitrate_estimator.rs#L76-91

In my local testing, I observed that the sample_var value was around 2000 and the self.estimate_var value was around 20. As a result, the BWE increased very slowly. For instance, if the estimated bitrate was 400 and the previous bitrate was 40, the BWE would only increase to 43.6, which is a mere 1% increase. After my configuration of a 2-second warm-up time ends, the video bitrate drops dramatically to around 80 kbps, causing video freezing and very poor quality.

Solution

This PR introduces a BWE reset API, which allows the BWE to be reset when the server starts sending video packets after an initial period of audio-only packets. This enables the BWE to quickly adapt to the changed bandwidth requirements, ensuring a smoother and more accurate bitrate estimation.