WebRTC Loss based Bandwidth Estimation

Abstract	WebRTC Probe Controller
Authors	Walter Fan
Status	WIP
Updated	2024-08-21

Overview

classes

我们需要考虑的因素很多, 什么时候增加/减小带宽, 增加/减小多少, 调整的频率, 调整的幅度都需要仔细考量.

输入因素:

1. 发送的比特率 sendBitrate

2. 请求的比特率 requestBitrate

3. 丢包率: packetLossRatio = (sendBitrate - receiveBitrate)/sendBitrate

4. 延迟: RTT 及 QueueDelay 排队延迟

5. 当前状态 overuse, underuse or normal 以及持续了多少时间

6. 之前状态, 以及切换到当前状态的时间

7. 上次调整的行为

8. 上次调整的时间

配置类

struct LossBasedControlConfig {
   explicit LossBasedControlConfig(const FieldTrialsView* key_value_config);
   LossBasedControlConfig(const LossBasedControlConfig&);
   LossBasedControlConfig& operator=(const LossBasedControlConfig&) = default;
   ~LossBasedControlConfig();
   bool enabled;
   FieldTrialParameter<double> min_increase_factor;
   FieldTrialParameter<double> max_increase_factor;
   FieldTrialParameter<TimeDelta> increase_low_rtt;
   FieldTrialParameter<TimeDelta> increase_high_rtt;
   FieldTrialParameter<double> decrease_factor;
   FieldTrialParameter<TimeDelta> loss_window;
   FieldTrialParameter<TimeDelta> loss_max_window;
   FieldTrialParameter<TimeDelta> acknowledged_rate_max_window;
   FieldTrialParameter<DataRate> increase_offset;
   FieldTrialParameter<DataRate> loss_bandwidth_balance_increase;
   FieldTrialParameter<DataRate> loss_bandwidth_balance_decrease;
   FieldTrialParameter<DataRate> loss_bandwidth_balance_reset;
   FieldTrialParameter<double> loss_bandwidth_balance_exponent;
   FieldTrialParameter<bool> allow_resets;
   FieldTrialParameter<TimeDelta> decrease_interval;
   FieldTrialParameter<TimeDelta> loss_report_timeout;
};

• 核心类

// Estimates an upper BWE limit based on loss.
// It requires knowledge about lost packets and acknowledged bitrate.
// Ie, this class require transport feedback.
class LossBasedBandwidthEstimation {
   public:
      explicit LossBasedBandwidthEstimation(
            const FieldTrialsView* key_value_config);
      // Returns the new estimate.
      DataRate Update(Timestamp at_time,
                        DataRate min_bitrate,
                        DataRate wanted_bitrate,
                        TimeDelta last_round_trip_time);
      void UpdateAcknowledgedBitrate(DataRate acknowledged_bitrate,
                                       Timestamp at_time);
      void Initialize(DataRate bitrate);
      bool Enabled() const { return config_.enabled; }
      // Returns true if LossBasedBandwidthEstimation is enabled and have
      // received loss statistics. Ie, this class require transport feedback.
      bool InUse() const {
         return Enabled() && last_loss_packet_report_.IsFinite();
      }
      void UpdateLossStatistics(const std::vector<PacketResult>& packet_results,
                                 Timestamp at_time);
      DataRate GetEstimate() const { return loss_based_bitrate_; }

   private:
      friend class GoogCcStatePrinter;
      void Reset(DataRate bitrate);
      double loss_increase_threshold() const;
      double loss_decrease_threshold() const;
      double loss_reset_threshold() const;

      DataRate decreased_bitrate() const;

      const LossBasedControlConfig config_;
      double average_loss_;
      double average_loss_max_;
      DataRate loss_based_bitrate_;
      DataRate acknowledged_bitrate_max_;
      Timestamp acknowledged_bitrate_last_update_;
      Timestamp time_last_decrease_;
      bool has_decreased_since_last_loss_report_;
      Timestamp last_loss_packet_report_;
      double last_loss_ratio_;
};