目录

背景介绍：

1.数据结构设计

2.对外提供的接口

3.核心处理逻辑

4.相关组件源码分析

4.1 BitrateProber

4.2 AlrDetector

4.3 IntervalBudget

5.总结

背景介绍：

TCP网络协议会维护拥塞窗口和发送窗口，在rtc中，也有类似的缓存机制，就是pacer模块，视频编码后的帧数据量都比较大，尤其是i帧 ，如果一股脑的往网络里直接发送会导致网络拥塞情况出现，如何根据拥塞估计反馈的码率完成优雅的平滑发送是pacer模块设计需要考虑的事情。

1.数据结构设计

class Pacer : public Module, public RtpPacketSender {
}


class PacedSender : public Pacer{
}

从上述代码看出来pacer类继承了Module模块，实现了相关定时处理接口，将由特定线程定时调用。pacersender是主要功能实现类，其重要成员如下：

kMaxQueueLengthMs //最大保存多少ms

static const float kDefaultPaceMultiplier; //一个因子系数？改变它可以降低缓冲延迟？

paused_ ：代表是否发送
const Clock* const clock_; //时钟
const std::unique_ptr<AlrDetector> alr_detector_ //判断当前输出网络流量是否受限 ，是否打开alr状态
//就是一段时间内的媒体发送码率预算。 IntervalBudget根据时间流逝增加budget，报文发送后减少budget，每次发送报文前判断剩余budget是否足够，如果不足则取消本次发送。
const std::unique_ptr<IntervalBudget> media_budget_

const std::unique_ptr<IntervalBudget> padding_budget_ //码率不足需要补padding包

const std::unique_ptr<BitrateProber> prober_ //带宽维持器  当码率不足时 需要开启这个来维持一定的码率发送 用于带宽探测

uint32_t estimated_bitrate_bps_  //估算带宽

uint32_t min_send_bitrate_kbps_ //最小带宽

uint32_t max_padding_bitrate_kbps_ //最大带宽

uint32_t pacing_bitrate_kbps_   //pacing 分配带宽

int64_t time_last_update_us_ //上次更新时间

int64_t first_sent_packet_ms_  //第一次发送时间

const std::unique_ptr<PacketQueue> packets_  //包缓存优先级队列

uint64_t packet_counter_ //包计数？

float pacing_factor_  //平滑发送系数

int64_t queue_time_limit //最大缓存时间

bool account_for_audio_ //是否开启音频流量统计 ，音频的packet具备最高优先级


class PacketQueue {
 public:
  explicit PacketQueue(const Clock* clock);
  virtual ~PacketQueue();

  struct Packet {
    Packet(RtpPacketSender::Priority priority,
           uint32_t ssrc,
           uint16_t seq_number,
           int64_t capture_time_ms,
           int64_t enqueue_time_ms,
           size_t length_in_bytes,
           bool retransmission,
           uint64_t enqueue_order);

    Packet(const Packet& other);

    virtual ~Packet();

    bool operator<(const Packet& other) const {
      if (priority != other.priority)
        return priority > other.priority;
      if (retransmission != other.retransmission)
        return other.retransmission;

      return enqueue_order > other.enqueue_order;
    }

    RtpPacketSender::Priority priority;
    uint32_t ssrc;
    uint16_t sequence_number;
    int64_t capture_time_ms;  // Absolute time of frame capture.
    int64_t enqueue_time_ms;  // Absolute time of pacer queue entry.
    int64_t sum_paused_ms;
    size_t bytes;
    bool retransmission;
    uint64_t enqueue_order;
    std::list<Packet>::iterator this_it;
    std::multiset<int64_t>::iterator enqueue_time_it;
  };

  virtual void Push(const Packet& packet);
  virtual const Packet& BeginPop();
  virtual void CancelPop(const Packet& packet);
  virtual void FinalizePop(const Packet& packet);
  virtual bool Empty() const;
  virtual size_t SizeInPackets() const;
  virtual uint64_t SizeInBytes() const;
  virtual int64_t OldestEnqueueTimeMs() const;
  virtual void UpdateQueueTime(int64_t timestamp_ms);
  virtual void SetPauseState(bool paused, int64_t timestamp_ms);
  virtual int64_t AverageQueueTimeMs() const;

 private:
  // Try to add a packet to the set of ssrc/seqno identifiers currently in the
  // queue. Return true if inserted, false if this is a duplicate.
  bool AddToDupeSet(const Packet& packet);

  void RemoveFromDupeSet(const Packet& packet);

  // Used by priority queue to sort packets.
  struct Comparator {
    bool operator()(const Packet* first, const Packet* second) {
      // Highest prio = 0.
      if (first->priority != second->priority)
        return first->priority > second->priority;

      // Retransmissions go first.
      if (second->retransmission != first->retransmission)
        return second->retransmission;

      // Older frames have higher prio.
      if (first->capture_time_ms != second->capture_time_ms)
        return first->capture_time_ms > second->capture_time_ms;

      return first->enqueue_order > second->enqueue_order;
    }
  };

  // List of packets, in the order the were enqueued. Since dequeueing may
  // occur out of order, use list instead of vector.
  std::list<Packet> packet_list_;        //存储packet 
  // Priority queue of the packets, sorted according to Comparator.
  // Use pointers into list, to avodi moving whole struct within heap.
  std::priority_queue<Packet*, std::vector<Packet*>, Comparator> prio_queue_;  //优先级队列，实际是存储的packet指针
  // Total number of bytes in the queue.
  uint64_t bytes_;
  const Clock* const clock_;
  int64_t queue_time_sum_;
  int64_t time_last_updated_;
  bool paused_;
};

2.对外提供的接口

virtual void CreateProbeCluster(int bitrate_bps);  //创建一个码率探测的probecluster


//暂停发送
void Pause();