Ulpfec_Receiver_AddReceivedRedPacket

bool UlpfecReceiver::AddReceivedRedPacket(const RtpPacketReceived& rtp_packet) {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  // TODO(bugs.webrtc.org/11993): We get here via Call::DeliverRtp, so should be
  // moved to the network thread.

  if (rtp_packet.Ssrc() != ssrc_) {
    RTC_LOG(LS_WARNING)
        << "Received RED packet with different SSRC than expected; dropping.";
    return false;
  }
  if (rtp_packet.size() > IP_PACKET_SIZE) {
    RTC_LOG(LS_WARNING) << "Received RED packet with length exceeds maximum IP "
                           "packet size; dropping.";
    return false;
  }

  static constexpr uint8_t kRedHeaderLength = 1;

  if (rtp_packet.payload_size() == 0) {
    RTC_LOG(LS_WARNING) << "Corrupt/truncated FEC packet.";
    return false;
  }

  // Remove RED header of incoming packet and store as a virtual RTP packet.
  auto received_packet =
      std::make_unique<ForwardErrorCorrection::ReceivedPacket>();
  received_packet->pkt = new ForwardErrorCorrection::Packet();

  // Get payload type from RED header and sequence number from RTP header.
  uint8_t payload_type = rtp_packet.payload()[0] & 0x7f;
  received_packet->is_fec = payload_type == ulpfec_payload_type_;
  received_packet->is_recovered = rtp_packet.recovered();
  received_packet->ssrc = rtp_packet.Ssrc();
  received_packet->seq_num = rtp_packet.SequenceNumber();
  received_packet->extensions = rtp_packet.extension_manager();

  if (rtp_packet.payload()[0] & 0x80) {
    // f bit set in RED header, i.e. there are more than one RED header blocks.
    // WebRTC never generates multiple blocks in a RED packet for FEC.
    RTC_LOG(LS_WARNING) << "More than 1 block in RED packet is not supported.";
    return false;
  }

  ++packet_counter_.num_packets;
  packet_counter_.num_bytes += rtp_packet.size();
  if (packet_counter_.first_packet_time == Timestamp::MinusInfinity()) {
    packet_counter_.first_packet_time = clock_->CurrentTime();
  }

  if (received_packet->is_fec) {
    ++packet_counter_.num_fec_packets;
    // everything behind the RED header
    received_packet->pkt->data =
        rtp_packet.Buffer().Slice(rtp_packet.headers_size() + kRedHeaderLength,
                                  rtp_packet.payload_size() - kRedHeaderLength);
  } else {
    received_packet->pkt->data.EnsureCapacity(rtp_packet.size() -
                                              kRedHeaderLength);
    // Copy RTP header.
    received_packet->pkt->data.SetData(rtp_packet.data(),
                                       rtp_packet.headers_size());
    // Set payload type.
    uint8_t& payload_type_byte = received_packet->pkt->data.MutableData()[1];
    payload_type_byte &= 0x80;          // Reset RED payload type.
    payload_type_byte += payload_type;  // Set media payload type.
    // Copy payload and padding data, after the RED header.
    received_packet->pkt->data.AppendData(
        rtp_packet.data() + rtp_packet.headers_size() + kRedHeaderLength,
        rtp_packet.size() - rtp_packet.headers_size() - kRedHeaderLength);
  }

  if (received_packet->pkt->data.size() > 0) {
    received_packets_.push_back(std::move(received_packet));
  }
  return true;
}

bool RTPSenderVideo::SendVideo(int payload_type,
                               std::optional<VideoCodecType> codec_type,
                               uint32_t rtp_timestamp,
                               Timestamp capture_time,
                               ArrayView<const uint8_t> payload,
                               size_t encoder_output_size,
                               RTPVideoHeader video_header,
                               TimeDelta expected_retransmission_time,
                               std::vector<uint32_t> csrcs) {
...
    if (red_enabled()) {
      // TODO(sprang): Consider packetizing directly into packets with the RED
      // header already in place, to avoid this copy.
      std::unique_ptr<RtpPacketToSend> red_packet(new RtpPacketToSend(*packet));
      BuildRedPayload(*packet, red_packet.get());
      red_packet->SetPayloadType(*red_payload_type_);
      red_packet->set_is_red(true);

      // Append `red_packet` instead of `packet` to output.
      red_packet->set_packet_type(RtpPacketMediaType::kVideo);
      red_packet->set_allow_retransmission(packet->allow_retransmission());
      rtp_packets.emplace_back(std::move(red_packet));
    } ß
...
}

AudioEncoder::EncodedInfo AudioEncoderCopyRed::EncodeImpl(
    uint32_t rtp_timestamp,
    ArrayView<const int16_t> audio,
    Buffer* encoded) {
  ...

  size_t header_offset = 0;
  while (it-- != redundant_encodings_.begin()) {
    encoded->AppendData(it->second);

    const uint32_t timestamp_delta =
        info.encoded_timestamp - it->first.encoded_timestamp;
    encoded->data()[header_offset] = it->first.payload_type | 0x80;
    SetBE16(static_cast<uint8_t*>(encoded->data()) + header_offset + 1,
            (timestamp_delta << 2) | (it->first.encoded_bytes >> 8));
    encoded->data()[header_offset + 3] = it->first.encoded_bytes & 0xff;
    header_offset += kRedHeaderLength;
    info.redundant.push_back(it->first);
  }
...
}