WebRTC NACK & FEC 配合策略

// Hybrid Nack FEC has three operational modes:
// 1. Low RTT (below kLowRttNackMs) - Nack only: Set FEC rate
// (_protectionFactorD) to zero. -1 means no FEC.
// 2. High RTT (above _highRttNackMs) - FEC Only: Keep FEC factors.
// -1 means always allow NACK.
// 3. Medium RTT values - Hybrid mode: We will only nack the
// residual following the decoding of the FEC (refer to JB logic). FEC
// delta protection factor will be adjusted based on the RTT.
// Otherwise: we count on FEC; if the RTT is below a threshold, then we
// nack the residual, based on a decision made in the JB.

WebRTC 中代码：

bool VCMNackFecMethod::ProtectionFactor(
    const VCMProtectionParameters* parameters) {
  // Hybrid Nack FEC has three operational modes:
  // 1. Low RTT (below kLowRttNackMs) - Nack only: Set FEC rate
  //    (_protectionFactorD) to zero. -1 means no FEC.
  // 2. High RTT (above _highRttNackMs) - FEC Only: Keep FEC factors.
  //    -1 means always allow NACK.
  // 3. Medium RTT values - Hybrid mode: We will only nack the
  //    residual following the decoding of the FEC (refer to JB logic). FEC
  //    delta protection factor will be adjusted based on the RTT.

  // Otherwise: we count on FEC; if the RTT is below a threshold, then we
  // nack the residual, based on a decision made in the JB.

  // Compute the protection factors
  VCMFecMethod::ProtectionFactor(parameters);
  if (_lowRttNackMs == -1 || parameters->rtt < _lowRttNackMs) {
    _protectionFactorD = 0;
    VCMFecMethod::UpdateProtectionFactorD(_protectionFactorD);

    // When in Hybrid mode (RTT range), adjust FEC rates based on the
    // RTT (NACK effectiveness) - adjustment factor is in the range [0,1].
  } else if (_highRttNackMs == -1 || parameters->rtt < _highRttNackMs) {
    // TODO(mikhal): Disabling adjustment temporarily.
    // uint16_t rttIndex = (uint16_t) parameters->rtt;
    float adjustRtt = 1.0f;  // (float)VCMNackFecTable[rttIndex] / 100.0f;

    // Adjust FEC with NACK on (for delta frame only)
    // table depends on RTT relative to rttMax (NACK Threshold)
    _protectionFactorD = rtc::saturated_cast<uint8_t>(
        adjustRtt * rtc::saturated_cast<float>(_protectionFactorD));
    // update FEC rates after applying adjustment
    VCMFecMethod::UpdateProtectionFactorD(_protectionFactorD);
  }

  return true;
}

VCMFecMethod::ProtectionFactor(const VCMProtectionParameters* parameters) 函数中，根据网络估计带宽，分配FEC带宽：

bool VCMNackFecMethod::UpdateParameters(
    const VCMProtectionParameters* parameters) {
  ProtectionFactor(parameters);
  EffectivePacketLoss(parameters);
  _maxFramesFec = ComputeMaxFramesFec(parameters);
  if (BitRateTooLowForFec(parameters)) {
    _protectionFactorK = 0;
    _protectionFactorD = 0;
  }

  // Protection/fec rates obtained above are defined relative to total number
  // of packets (total rate: source + fec) FEC in RTP module assumes
  // protection factor is defined relative to source number of packets so we
  // should convert the factor to reduce mismatch between mediaOpt's rate and
  // the actual one
  _protectionFactorK = VCMFecMethod::ConvertFECRate(_protectionFactorK);
// 主要是此系数D
  _protectionFactorD = VCMFecMethod::ConvertFECRate(_protectionFactorD);
}

根据丢包率，计算 _protectionFactorK, kPacketLossMax 最大为 127（按[0 - 255]）计算

  // Check limit on amount of protection for P frame; 50% is max.
  if (codeRateDelta >= kPacketLossMax) {
    codeRateDelta = kPacketLossMax - 1;
  }

  // For Key frame:
  // Effectively at a higher rate, so we scale/boost the rate
  // The boost factor may depend on several factors: ratio of packet
  // number of I to P frames, how much protection placed on P frames, etc.
  const uint8_t packetFrameDelta =
      rtc::saturated_cast<uint8_t>(0.5 + parameters->packetsPerFrame);
  const uint8_t packetFrameKey =
      rtc::saturated_cast<uint8_t>(0.5 + parameters->packetsPerFrameKey);
  const uint8_t boostKey = BoostCodeRateKey(packetFrameDelta, packetFrameKey);

  rateIndexTable = rtc::saturated_cast<uint8_t>(VCM_MAX(
      VCM_MIN(1 + (boostKey * effRateFecTable - ratePar1) / ratePar1, ratePar2),
      0));
  uint16_t indexTableKey = rateIndexTable * kPacketLossMax + packetLoss;

  indexTableKey = VCM_MIN(indexTableKey, kFecRateTableSize);

  // Check on table index
  assert(indexTableKey < kFecRateTableSize);

  // Protection factor for I frame
  codeRateKey = kFecRateTable[indexTableKey];

  // Boosting for Key frame.
  int boostKeyProt = _scaleProtKey * codeRateDelta;
  if (boostKeyProt >= kPacketLossMax) {
    boostKeyProt = kPacketLossMax - 1;
  }

  // Make sure I frame protection is at least larger than P frame protection,
  // and at least as high as filtered packet loss.
  codeRateKey = rtc::saturated_cast<uint8_t>(
      VCM_MAX(packetLoss, VCM_MAX(boostKeyProt, codeRateKey)));

  // Check limit on amount of protection for I frame: 50% is max.
  if (codeRateKey >= kPacketLossMax) {
    codeRateKey = kPacketLossMax - 1;
  }

  _protectionFactorK = codeRateKey;
  _protectionFactorD = codeRateDelta;++

FecControllerDefault::UpdateFecRates() 调用VCMProtectionMethod::RequiredProtectionFactorD方法更新 fec_rate, 并callback 到 RtpVideoSender

delta_fec_params.fec_rate =
        loss_prot_logic_->SelectedMethod()->RequiredProtectionFactorD();

<-------------------------->
protection_callback_->ProtectionRequest(
      &delta_fec_params, &key_fec_params, &sent_video_rate_bps,
      &sent_nack_rate_bps, &sent_fec_rate_bps);

之后 stream.fec_generator 根据params中fec码率生成FEC包

int RtpVideoSender::ProtectionRequest(const FecProtectionParams* delta_params,
                                      const FecProtectionParams* key_params,
                                      uint32_t* sent_video_rate_bps,
                                      uint32_t* sent_nack_rate_bps,
                                      uint32_t* sent_fec_rate_bps) {
  *sent_video_rate_bps = 0;
  *sent_nack_rate_bps = 0;
  *sent_fec_rate_bps = 0;
  for (const RtpStreamSender& stream : rtp_streams_) {
    if (stream.fec_generator) {
      stream.fec_generator->SetProtectionParameters(*delta_params, *key_params);
      *sent_fec_rate_bps += stream.fec_generator->CurrentFecRate().bps();
    }
    *sent_video_rate_bps += stream.sender_video->VideoBitrateSent();
    *sent_nack_rate_bps +=
        stream.rtp_rtcp->GetSendRates()[RtpPacketMediaType::kRetransmission]
            .bps<uint32_t>();
  }
  return 0;
}