WebRTC研究：丢包判断

/*
sequence_number：当前收到的包***
*/
bool VCMJitterBuffer::UpdateNackList(uint16_t sequence_number) {
  if (nack_mode_ == kNoNack) {
    return true;
  }

  // Make sure we don't add packets which are already too old to be decoded.
  if (!last_decoded_state_.in_initial_state()) {

	  /* latest_received_sequence_number_：上一次收到的最新RTP包*** */
    latest_received_sequence_number_ = LatestSequenceNumber(
        latest_received_sequence_number_, last_decoded_state_.sequence_num());
  }

  /* 判断当前RTP包是否是新的数据包，而不是重传或者乱序包 */
  if (IsNewerSequenceNumber(sequence_number, latest_received_sequence_number_)) 
  {
	/* 
	从 latest_received_sequence_number_ + 1 开始判断是否满足 IsNewerSequenceNumber，
	即：判断当前包的*** sequence_number 是不是比排在 i 的后面，
	是的话，说明 [latest_received_sequence_number_ + 1, latest_received_sequence_number_] 区间的包丢了
	*/
    for (uint16_t i = latest_received_sequence_number_ + 1; IsNewerSequenceNumber(sequence_number, i); ++i) {

	  /* 将*** i 添加到 missing_sequence_numbers_（std::set类型），被认为包已经丢失 */
      missing_sequence_numbers_.insert(missing_sequence_numbers_.end(), i);
      TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("webrtc_rtp"), "AddNack", "seqnum", i);
    }

	/*
	判断集合 missing_sequence_numbers_ 的容量是否超过最大限制
	是的话，通过调用 RecycleFramesUntilKeyFrame 不断丢包来减少集合中的***
	*/
    if (TooLargeNackList() && !HandleTooLargeNackList()) 
	{
      LOG(LS_WARNING) << "Requesting key frame due to too large NACK list.";
      return false;
    }

	/*
	判断集合 missing_sequence_numbers_ 里的是否太久了
	判断依据：当前包的*** sequence_number 与 集合 missing_sequence_numbers_ 中第一个包的***之间的差值是否超过限制
	是的话，通过调用 RecycleFramesUntilKeyFrame 不断丢包来减少集合中的***
	*/
    if (MissingTooOldPacket(sequence_number) && !HandleTooOldPackets(sequence_number)) {
      LOG(LS_WARNING) << "Requesting key frame due to missing too old packets";
      return false;
    }
  } 
  else 
  {
	/* 当前包是重传或者乱序包，直接从丢包集合中剔除 */
    missing_sequence_numbers_.erase(sequence_number);
    TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("webrtc_rtp"), "RemoveNack",
                         "seqnum", sequence_number);
  }

  return true;
}