问题

• 什么时候开始探测
• 什么时候停止探测
• 探测多长时间
• 以多大的比特率探测
• 以什么内容进行控制

答案

让我们从代码中寻找答案吧

• ProbeController

• ProbeControllerConfig

第一次 p1 * start_bitrate_bps

struct ProbeControllerConfig {
  explicit ProbeControllerConfig(const FieldTrialsView* key_value_config);
  ProbeControllerConfig(const ProbeControllerConfig&);
  ProbeControllerConfig& operator=(const ProbeControllerConfig&) = default;
  ~ProbeControllerConfig();

  // These parameters configure the initial probes. First we send one or two
  // probes of sizes p1 * start_bitrate_bps_ and p2 * start_bitrate_bps_.
  // Then whenever we get a bitrate estimate of at least further_probe_threshold
  // times the size of the last sent probe we'll send another one of size
  // step_size times the new estimate.
  FieldTrialParameter<double> first_exponential_probe_scale;
  FieldTrialOptional<double> second_exponential_probe_scale;
  FieldTrialParameter<double> further_exponential_probe_scale;
  FieldTrialParameter<double> further_probe_threshold;

  // Configures how often we send ALR probes and how big they are.
  FieldTrialParameter<TimeDelta> alr_probing_interval;
  FieldTrialParameter<double> alr_probe_scale;

  // Configures the probes emitted by changed to the allocated bitrate.
  FieldTrialOptional<double> first_allocation_probe_scale;
  FieldTrialOptional<double> second_allocation_probe_scale;
  FieldTrialFlag allocation_allow_further_probing;
  FieldTrialParameter<DataRate> allocation_probe_max;

  // The minimum number probing packets used.
  FieldTrialParameter<int> min_probe_packets_sent;
  // The minimum probing duration.
  FieldTrialParameter<TimeDelta> min_probe_duration;
};

• ProbeController::State

  enum class State {
    // Initial state where no probing has been triggered yet.
    kInit,
    // Waiting for probing results to continue further probing.
    kWaitingForProbingResult,
    // Probing is complete.
    kProbingComplete,
  };

• ProbeClusterConfig

struct ProbeClusterConfig {
  Timestamp at_time = Timestamp::PlusInfinity();
  DataRate target_data_rate = DataRate::Zero();
  TimeDelta target_duration = TimeDelta::Zero();
  int32_t target_probe_count = 0;
  int32_t id = 0;
};

ProbeController 属性

// This class controls initiation of probing to estimate initial channel
// capacity. There is also support for probing during a session when max
// bitrate is adjusted by an application.
class ProbeController {
 //...
  bool network_available_;
  State state_;
  int64_t min_bitrate_to_probe_further_bps_;
  int64_t time_last_probing_initiated_ms_;
  int64_t estimated_bitrate_bps_;
  int64_t start_bitrate_bps_;
  int64_t max_bitrate_bps_;
  int64_t last_bwe_drop_probing_time_ms_;
  absl::optional<int64_t> alr_start_time_ms_;
  absl::optional<int64_t> alr_end_time_ms_;
  bool enable_periodic_alr_probing_;
  int64_t time_of_last_large_drop_ms_;
  int64_t bitrate_before_last_large_drop_bps_;
  int64_t max_total_allocated_bitrate_;

  const bool in_rapid_recovery_experiment_;
  const bool limit_probes_with_allocateable_rate_;
  // For WebRTC.BWE.MidCallProbing.* metric.
  bool mid_call_probing_waiting_for_result_;
  int64_t mid_call_probing_bitrate_bps_;
  int64_t mid_call_probing_succcess_threshold_;
  RtcEventLog* event_log_;

  int32_t next_probe_cluster_id_ = 1;

  ProbeControllerConfig config_;
};