PacketBuffer

packetbuffer类中重要的一些变量

 // buffer_.size() and max_size_ must always be a power of two.
  const size_t max_size_;//能存储的最大元素个数

  // The fist sequence number currently in the buffer.
  uint16_t first_seq_num_ RTC_GUARDED_BY(crit_);//这个packetbuffer所保存的第一包的seqnumber

  // If the packet buffer has received its first packet.
  bool first_packet_received_ RTC_GUARDED_BY(crit_);//是否已经接受过数据

  // If the buffer is cleared to |first_seq_num_|.
  bool is_cleared_to_first_seq_num_ RTC_GUARDED_BY(crit_);

  // Buffer that holds the the inserted packets and information needed to
  // determine continuity between them.
  std::vector<std::unique_ptr<Packet>> buffer_ RTC_GUARDED_BY(crit_);//保存缓冲的RTP包

  // Timestamp of the last received packet/keyframe packet.
  absl::optional<int64_t> last_received_packet_