WebRTC源码分析--RateLimiter

RateLimiter是webrtc中实现的限流工具，被用在诸如限制重传包数量等地方。它依赖于之前介绍的速率计算器RateStatistics。

先看它的定义：

class RateLimiter {
 public:
  RateLimiter(Clock* clock, int64_t max_window_ms);
  ~RateLimiter();

  // Try to use rate to send bytes. Returns true on success and if so updates
  // current rate.
  bool TryUseRate(size_t packet_size_bytes);

  // Set the maximum bitrate, in bps, that this limiter allows to send.
  void SetMaxRate(uint32_t max_rate_bps);

  // Set the window size over which to measure the current bitrate.
  // For example, irt retransmissions, this is typically the RTT.
  // Returns true on success and false if window_size_ms is out of range.
  bool SetWindowSize(int64_t window_size_ms);

 private:
  Clock* const clock_;
  rtc::CriticalSection lock_;
  RateStatistics current_rate_ RTC_GUARDED_BY(lock_);
  int64_t window_size_ms_ RTC_GUARDED_BY(lock_);
  uint32_t max_rate_bps_ RTC_GUARDED_BY(lock_);

  RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(RateLimiter);
};

接口十分简单：

• TryUseRate: 尝试使用packet_size_bytes字节的数据，该接口基于当前状态返回是否允许，这是主要的接口；
• SetMaxRate： 设置允许的最大速率，即上限；
• SetWindowSize：设置统计速率的时间窗口，也是设置RateStatistics的窗口。

SetMaxRate和SetWindowSize都比较简单，只是设置参数：

void RateLimiter::SetMaxRate(uint32_t max_rate_bps) {
  rtc::CritScope cs(&lock_);
  max_rate_bps_ = max_rate_bps;
}

// Set the window size over which to measure the current bitrate.
// For retransmissions, this is typically the RTT.
bool RateLimiter::SetWindowSize(int64_t window_size_ms) {
  rtc::CritScope cs(&lock_);
  window_size_ms_ = window_size_ms;
  return current_rate_.SetWindowSize(window_size_ms,
                                     clock_->TimeInMilliseconds());
}

RateLimiter::TryUseRate是核心，用于比较当前的使用速率和预设的最大速率之间的关系：

bool RateLimiter::TryUseRate(size_t packet_size_bytes) {
  rtc::CritScope cs(&lock_);
  int64_t now_ms = clock_->TimeInMilliseconds();
  // 得到由RateStatistics计算得到的当前速率
  absl::optional<uint32_t> current_rate = current_rate_.Rate(now_ms);
  if (current_rate) {
    // If there is a current rate, check if adding bytes would cause maximum
    // bitrate target to be exceeded. If there is NOT a valid current rate,
    // allow allocating rate even if target is exceeded. This prevents
    // problems
    // at very low rates, where for instance retransmissions would never be
    // allowed due to too high bitrate caused by a single packet.

    // 如果使用当前packet_size_bytes，计算它占用的速率，转换层bps
    size_t bitrate_addition_bps =
        (packet_size_bytes * 8 * 1000) / window_size_ms_;
    // 判断是否超过预设值
    if (*current_rate + bitrate_addition_bps > max_rate_bps_)
      return false;
  }

  // 更新速率RateStatistics
  current_rate_.Update(packet_size_bytes, now_ms);
  return true;
}

• 通常SetWindowSize会被设置成一个rtt，并限定在一个预设范围内；
• 通常SetMaxRate会根据码率估计得到的值设置。