modules/rtp_rtcp/source/forward_error_correction.cc

/*
 *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#include "webrtc/modules/rtp_rtcp/source/forward_error_correction.h"

#include <string.h>

#include <algorithm>
#include <iterator>
#include <utility>

#include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
#include "webrtc/modules/rtp_rtcp/source/byte_io.h"
#include "webrtc/modules/rtp_rtcp/source/flexfec_header_reader_writer.h"
#include "webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h"
#include "webrtc/modules/rtp_rtcp/source/ulpfec_header_reader_writer.h"
#include "webrtc/rtc_base/checks.h"
#include "webrtc/rtc_base/logging.h"

namespace webrtc {

namespace {
// Transport header size in bytes. Assume UDP/IPv4 as a reasonable minimum.
constexpr size_t kTransportOverhead = 28;
}  // namespace

ForwardErrorCorrection::Packet::Packet() : length(0), data(), ref_count_(0) {}
ForwardErrorCorrection::Packet::~Packet() = default;

int32_t ForwardErrorCorrection::Packet::AddRef() {
  return ++ref_count_;
}

int32_t ForwardErrorCorrection::Packet::Release() {
  int32_t ref_count;
  ref_count = --ref_count_;
  if (ref_count == 0)
    delete this;
  return ref_count;
}

// This comparator is used to compare std::unique_ptr's pointing to
// subclasses of SortablePackets. It needs to be parametric since
// the std::unique_ptr's are not covariant w.r.t. the types that
// they are pointing to.
template <typename S, typename T>
bool ForwardErrorCorrection::SortablePacket::LessThan::operator() (
    const S& first,
    const T& second) {
  RTC_DCHECK_EQ(first->ssrc, second->ssrc);
  return IsNewerSequenceNumber(second->seq_num, first->seq_num);
}

ForwardErrorCorrection::ReceivedPacket::ReceivedPacket() = default;
ForwardErrorCorrection::ReceivedPacket::~ReceivedPacket() = default;

ForwardErrorCorrection::RecoveredPacket::RecoveredPacket() = default;
ForwardErrorCorrection::RecoveredPacket::~RecoveredPacket() = default;

ForwardErrorCorrection::ProtectedPacket::ProtectedPacket() = default;
ForwardErrorCorrection::ProtectedPacket::~ProtectedPacket() = default;

ForwardErrorCorrection::ReceivedFecPacket::ReceivedFecPacket() = default;
ForwardErrorCorrection::ReceivedFecPacket::~ReceivedFecPacket() = default;

ForwardErrorCorrection::ForwardErrorCorrection(
    std::unique_ptr<FecHeaderReader> fec_header_reader,
    std::unique_ptr<FecHeaderWriter> fec_header_writer,
    uint32_t ssrc,
    uint32_t protected_media_ssrc)
    : ssrc_(ssrc),
      protected_media_ssrc_(protected_media_ssrc),
      fec_header_reader_(std::move(fec_header_reader)),
      fec_header_writer_(std::move(fec_header_writer)),
      generated_fec_packets_(fec_header_writer_->MaxFecPackets()),
      packet_mask_size_(0) {}

ForwardErrorCorrection::~ForwardErrorCorrection() = default;

std::unique_ptr<ForwardErrorCorrection> ForwardErrorCorrection::CreateUlpfec(
    uint32_t ssrc) {
  std::unique_ptr<FecHeaderReader> fec_header_reader(new UlpfecHeaderReader());
  std::unique_ptr<FecHeaderWriter> fec_header_writer(new UlpfecHeaderWriter());
  return std::unique_ptr<ForwardErrorCorrection>(new ForwardErrorCorrection(
      std::move(fec_header_reader), std::move(fec_header_writer), ssrc, ssrc));
}

std::unique_ptr<ForwardErrorCorrection> ForwardErrorCorrection::CreateFlexfec(
    uint32_t ssrc,
    uint32_t protected_media_ssrc) {
  std::unique_ptr<FecHeaderReader> fec_header_reader(new FlexfecHeaderReader());
  std::unique_ptr<FecHeaderWriter> fec_header_writer(new FlexfecHeaderWriter());
  return std::unique_ptr<ForwardErrorCorrection>(new ForwardErrorCorrection(
      std::move(fec_header_reader), std::move(fec_header_writer), ssrc,
      protected_media_ssrc));
}

int ForwardErrorCorrection::EncodeFec(const PacketList& media_packets,
                                      uint8_t protection_factor,
                                      int num_important_packets,
                                      bool use_unequal_protection,
                                      FecMaskType fec_mask_type,
                                      std::list<Packet*>* fec_packets) {
  const size_t num_media_packets = media_packets.size();

  // Sanity check arguments.
  RTC_DCHECK_GT(num_media_packets, 0);
  RTC_DCHECK_GE(num_important_packets, 0);
  RTC_DCHECK_LE(num_important_packets, num_media_packets);
  RTC_DCHECK(fec_packets->empty());
  const size_t max_media_packets = fec_header_writer_->MaxMediaPackets();
  if (num_media_packets > max_media_packets) {
    LOG(LS_WARNING) << "Can't protect " << num_media_packets
                    << " media packets per frame. Max is " << max_media_packets
                    << ".";
    return -1;
  }

  // Error check the media packets.
  for (const auto& media_packet : media_packets) {
    RTC_DCHECK(media_packet);
    if (media_packet->length < kRtpHeaderSize) {
      LOG(LS_WARNING) << "Media packet " << media_packet->length << " bytes "
                      << "is smaller than RTP header.";
      return -1;
    }
    // Ensure the FEC packets will fit in a typical MTU.
    if (media_packet->length + MaxPacketOverhead() + kTransportOverhead >
        IP_PACKET_SIZE) {
      LOG(LS_WARNING) << "Media packet " << media_packet->length << " bytes "
                      << "with overhead is larger than " << IP_PACKET_SIZE
                      << " bytes.";
    }
  }

  // Prepare generated FEC packets.
  int num_fec_packets = NumFecPackets(num_media_packets, protection_factor);
  if (num_fec_packets == 0) {
    return 0;
  }
  for (int i = 0; i < num_fec_packets; ++i) {
    memset(generated_fec_packets_[i].data, 0, IP_PACKET_SIZE);
    // Use this as a marker for untouched packets.
    generated_fec_packets_[i].length = 0;
    fec_packets->push_back(&generated_fec_packets_[i]);
  }

  const internal::PacketMaskTable mask_table(fec_mask_type, num_media_packets);
  packet_mask_size_ = internal::PacketMaskSize(num_media_packets);
  memset(packet_masks_, 0, num_fec_packets * packet_mask_size_);
  internal::GeneratePacketMasks(num_media_packets, num_fec_packets,
                                num_important_packets, use_unequal_protection,
                                mask_table, packet_masks_);

  // Adapt packet masks to missing media packets.
  int num_mask_bits = InsertZerosInPacketMasks(media_packets, num_fec_packets);
  if (num_mask_bits < 0) {
    return -1;
  }
  packet_mask_size_ = internal::PacketMaskSize(num_mask_bits);

  // Write FEC packets to |generated_fec_packets_|.
  GenerateFecPayloads(media_packets, num_fec_packets);
  // TODO(brandtr): Generalize this when multistream protection support is
  // added.
  const uint32_t media_ssrc = ParseSsrc(media_packets.front()->data);
  const uint16_t seq_num_base =
      ParseSequenceNumber(media_packets.front()->data);
  FinalizeFecHeaders(num_fec_packets, media_ssrc, seq_num_base);

  return 0;
}

int ForwardErrorCorrection::NumFecPackets(int num_media_packets,
                                          int protection_factor) {
  // Result in Q0 with an unsigned round.
  int num_fec_packets = (num_media_packets * protection_factor + (1 << 7)) >> 8;
  // Generate at least one FEC packet if we need protection.
  if (protection_factor > 0 && num_fec_packets == 0) {
    num_fec_packets = 1;
  }
  RTC_DCHECK_LE(num_fec_packets, num_media_packets);
  return num_fec_packets;
}

void ForwardErrorCorrection::GenerateFecPayloads(
    const PacketList& media_packets,
    size_t num_fec_packets) {
  RTC_DCHECK(!media_packets.empty());
  for (size_t i = 0; i < num_fec_packets; ++i) {
    Packet* const fec_packet = &generated_fec_packets_[i];
    size_t pkt_mask_idx = i * packet_mask_size_;
    const size_t min_packet_mask_size = fec_header_writer_->MinPacketMaskSize(
        &packet_masks_[pkt_mask_idx], packet_mask_size_);
    const size_t fec_header_size =
        fec_header_writer_->FecHeaderSize(min_packet_mask_size);

    size_t media_pkt_idx = 0;
    auto media_packets_it = media_packets.cbegin();
    uint16_t prev_seq_num = ParseSequenceNumber((*media_packets_it)->data);
    while (media_packets_it != media_packets.end()) {
      Packet* const media_packet = media_packets_it->get();
      // Should |media_packet| be protected by |fec_packet|?
      if (packet_masks_[pkt_mask_idx] & (1 << (7 - media_pkt_idx))) {
        size_t media_payload_length = media_packet->length - kRtpHeaderSize;

        bool first_protected_packet = (fec_packet->length == 0);
        size_t fec_packet_length = fec_header_size + media_payload_length;
        if (fec_packet_length > fec_packet->length) {
          // Recall that XORing with zero (which the FEC packets are prefilled
          // with) is the identity operator, thus all prior XORs are
          // still correct even though we expand the packet length here.
          fec_packet->length = fec_packet_length;
        }
        if (first_protected_packet) {
          // Write P, X, CC, M, and PT recovery fields.
          // Note that bits 0, 1, and 16 are overwritten in FinalizeFecHeaders.
          memcpy(&fec_packet->data[0], &media_packet->data[0], 2);
          // Write length recovery field. (This is a temporary location for
          // ULPFEC.)
          ByteWriter<uint16_t>::WriteBigEndian(&fec_packet->data[2],
                                               media_payload_length);
          // Write timestamp recovery field.
          memcpy(&fec_packet->data[4], &media_packet->data[4], 4);
          // Write payload.
          memcpy(&fec_packet->data[fec_header_size],
                 &media_packet->data[kRtpHeaderSize], media_payload_length);
        } else {
          XorHeaders(*media_packet, fec_packet);
          XorPayloads(*media_packet, media_payload_length, fec_header_size,
                      fec_packet);
        }
      }
      media_packets_it++;
      if (media_packets_it != media_packets.end()) {
        uint16_t seq_num = ParseSequenceNumber((*media_packets_it)->data);
        media_pkt_idx += static_cast<uint16_t>(seq_num - prev_seq_num);
        prev_seq_num = seq_num;
      }
      pkt_mask_idx += media_pkt_idx / 8;
      media_pkt_idx %= 8;
    }
    RTC_DCHECK_GT(fec_packet->length, 0)
        << "Packet mask is wrong or poorly designed.";
  }
}

int ForwardErrorCorrection::InsertZerosInPacketMasks(
    const PacketList& media_packets,
    size_t num_fec_packets) {
  size_t num_media_packets = media_packets.size();
  if (num_media_packets <= 1) {
    return num_media_packets;
  }
  uint16_t last_seq_num = ParseSequenceNumber(media_packets.back()->data);
  uint16_t first_seq_num = ParseSequenceNumber(media_packets.front()->data);
  size_t total_missing_seq_nums =
      static_cast<uint16_t>(last_seq_num - first_seq_num) - num_media_packets +
      1;
  if (total_missing_seq_nums == 0) {
    // All sequence numbers are covered by the packet mask.
    // No zero insertion required.
    return num_media_packets;
  }
  const size_t max_media_packets = fec_header_writer_->MaxMediaPackets();
  if (total_missing_seq_nums + num_media_packets > max_media_packets) {
    return -1;
  }
  // Allocate the new mask.
  size_t tmp_packet_mask_size =
      internal::PacketMaskSize(total_missing_seq_nums + num_media_packets);
  memset(tmp_packet_masks_, 0, num_fec_packets * tmp_packet_mask_size);

  auto media_packets_it = media_packets.cbegin();
  uint16_t prev_seq_num = first_seq_num;
  ++media_packets_it;

  // Insert the first column.
  internal::CopyColumn(tmp_packet_masks_, tmp_packet_mask_size, packet_masks_,
                       packet_mask_size_, num_fec_packets, 0, 0);
  size_t new_bit_index = 1;
  size_t old_bit_index = 1;
  // Insert zeros in the bit mask for every hole in the sequence.
  while (media_packets_it != media_packets.end()) {
    if (new_bit_index == max_media_packets) {
      // We can only cover up to 48 packets.
      break;
    }
    uint16_t seq_num = ParseSequenceNumber((*media_packets_it)->data);
    const int num_zeros_to_insert =
        static_cast<uint16_t>(seq_num - prev_seq_num - 1);
    if (num_zeros_to_insert > 0) {
      internal::InsertZeroColumns(num_zeros_to_insert, tmp_packet_masks_,
                                  tmp_packet_mask_size, num_fec_packets,
                                  new_bit_index);
    }
    new_bit_index += num_zeros_to_insert;
    internal::CopyColumn(tmp_packet_masks_, tmp_packet_mask_size, packet_masks_,
                         packet_mask_size_, num_fec_packets, new_bit_index,
                         old_bit_index);
    ++new_bit_index;
    ++old_bit_index;
    prev_seq_num = seq_num;
    ++media_packets_it;
  }
  if (new_bit_index % 8 != 0) {
    // We didn't fill the last byte. Shift bits to correct position.
    for (uint16_t row = 0; row < num_fec_packets; ++row) {
      int new_byte_index = row * tmp_packet_mask_size + new_bit_index / 8;
      tmp_packet_masks_[new_byte_index] <<= (7 - (new_bit_index % 8));
    }
  }
  // Replace the old mask with the new.
  memcpy(packet_masks_, tmp_packet_masks_,
         num_fec_packets * tmp_packet_mask_size);
  return new_bit_index;
}

void ForwardErrorCorrection::FinalizeFecHeaders(size_t num_fec_packets,
                                                uint32_t media_ssrc,
                                                uint16_t seq_num_base) {
  for (size_t i = 0; i < num_fec_packets; ++i) {
    fec_header_writer_->FinalizeFecHeader(
        media_ssrc, seq_num_base, &packet_masks_[i * packet_mask_size_],
        packet_mask_size_, &generated_fec_packets_[i]);
  }
}

void ForwardErrorCorrection::ResetState(
    RecoveredPacketList* recovered_packets) {
  // Free the memory for any existing recovered packets, if the caller hasn't.
  recovered_packets->clear();
  received_fec_packets_.clear();
}

void ForwardErrorCorrection::InsertMediaPacket(
    RecoveredPacketList* recovered_packets,
    ReceivedPacket* received_packet) {
  RTC_DCHECK_EQ(received_packet->ssrc, protected_media_ssrc_);

  // Search for duplicate packets.
  for (const auto& recovered_packet : *recovered_packets) {
    RTC_DCHECK_EQ(recovered_packet->ssrc, received_packet->ssrc);
    if (recovered_packet->seq_num == received_packet->seq_num) {
      // Duplicate packet, no need to add to list.
      // Delete duplicate media packet data.
      received_packet->pkt = nullptr;
      return;
    }
  }

  std::unique_ptr<RecoveredPacket> recovered_packet(new RecoveredPacket());
  // This "recovered packet" was not recovered using parity packets.
  recovered_packet->was_recovered = false;
  // This media packet has already been passed on.
  recovered_packet->returned = true;
  recovered_packet->ssrc = received_packet->ssrc;
  recovered_packet->seq_num = received_packet->seq_num;
  recovered_packet->pkt = received_packet->pkt;
  recovered_packet->pkt->length = received_packet->pkt->length;
  // TODO(holmer): Consider replacing this with a binary search for the right
  // position, and then just insert the new packet. Would get rid of the sort.
  RecoveredPacket* recovered_packet_ptr = recovered_packet.get();
  recovered_packets->push_back(std::move(recovered_packet));
  recovered_packets->sort(SortablePacket::LessThan());
  UpdateCoveringFecPackets(*recovered_packet_ptr);
}

void ForwardErrorCorrection::UpdateCoveringFecPackets(
    const RecoveredPacket& packet) {
  for (auto& fec_packet : received_fec_packets_) {
    // Is this FEC packet protecting the media packet |packet|?
    auto protected_it = std::lower_bound(fec_packet->protected_packets.begin(),
                                         fec_packet->protected_packets.end(),
                                         &packet, SortablePacket::LessThan());
    if (protected_it != fec_packet->protected_packets.end() &&
        (*protected_it)->seq_num == packet.seq_num) {
      // Found an FEC packet which is protecting |packet|.
      (*protected_it)->pkt = packet.pkt;
    }
  }
}

void ForwardErrorCorrection::InsertFecPacket(
    const RecoveredPacketList& recovered_packets,
    ReceivedPacket* received_packet) {
  RTC_DCHECK_EQ(received_packet->ssrc, ssrc_);

  // Check for duplicate.
  for (const auto& existing_fec_packet : received_fec_packets_) {
    RTC_DCHECK_EQ(existing_fec_packet->ssrc, received_packet->ssrc);
    if (existing_fec_packet->seq_num == received_packet->seq_num) {
      // Delete duplicate FEC packet data.
      received_packet->pkt = nullptr;
      return;
    }
  }

  std::unique_ptr<ReceivedFecPacket> fec_packet(new ReceivedFecPacket());
  fec_packet->pkt = received_packet->pkt;
  fec_packet->ssrc = received_packet->ssrc;
  fec_packet->seq_num = received_packet->seq_num;
  // Parse ULPFEC/FlexFEC header specific info.
  bool ret = fec_header_reader_->ReadFecHeader(fec_packet.get());
  if (!ret) {
    return;
  }

  // TODO(brandtr): Update here when we support multistream protection.
  if (fec_packet->protected_ssrc != protected_media_ssrc_) {
    LOG(LS_INFO)
        << "Received FEC packet is protecting an unknown media SSRC; dropping.";
    return;
  }

  // Parse packet mask from header and represent as protected packets.
  for (uint16_t byte_idx = 0; byte_idx < fec_packet->packet_mask_size;
       ++byte_idx) {
    uint8_t packet_mask =
        fec_packet->pkt->data[fec_packet->packet_mask_offset + byte_idx];
    for (uint16_t bit_idx = 0; bit_idx < 8; ++bit_idx) {
      if (packet_mask & (1 << (7 - bit_idx))) {
        std::unique_ptr<ProtectedPacket> protected_packet(
            new ProtectedPacket());
        // This wraps naturally with the sequence number.
        protected_packet->ssrc = protected_media_ssrc_;
        protected_packet->seq_num = static_cast<uint16_t>(
            fec_packet->seq_num_base + (byte_idx << 3) + bit_idx);
        protected_packet->pkt = nullptr;
        fec_packet->protected_packets.push_back(std::move(protected_packet));
      }
    }
  }

  if (fec_packet->protected_packets.empty()) {
    // All-zero packet mask; we can discard this FEC packet.
    LOG(LS_WARNING) << "Received FEC packet has an all-zero packet mask.";
  } else {
    AssignRecoveredPackets(recovered_packets, fec_packet.get());
    // TODO(holmer): Consider replacing this with a binary search for the right
    // position, and then just insert the new packet. Would get rid of the sort.
    received_fec_packets_.push_back(std::move(fec_packet));
    received_fec_packets_.sort(SortablePacket::LessThan());
    const size_t max_fec_packets = fec_header_reader_->MaxFecPackets();
    if (received_fec_packets_.size() > max_fec_packets) {
      received_fec_packets_.pop_front();
    }
    RTC_DCHECK_LE(received_fec_packets_.size(), max_fec_packets);
  }
}

void ForwardErrorCorrection::AssignRecoveredPackets(
    const RecoveredPacketList& recovered_packets,
    ReceivedFecPacket* fec_packet) {
  ProtectedPacketList* protected_packets = &fec_packet->protected_packets;
  std::vector<RecoveredPacket*> recovered_protected_packets;

  // Find intersection between the (sorted) containers |protected_packets|
  // and |recovered_packets|, i.e. all protected packets that have already
  // been recovered. Update the corresponding protected packets to point to
  // the recovered packets.
  auto it_p = protected_packets->cbegin();
  auto it_r = recovered_packets.cbegin();
  SortablePacket::LessThan less_than;
  while (it_p != protected_packets->end() && it_r != recovered_packets.end()) {
    if (less_than(*it_p, *it_r)) {
      ++it_p;
    } else if (less_than(*it_r, *it_p)) {
      ++it_r;
    } else {  // *it_p == *it_r.
      // This protected packet has already been recovered.
      (*it_p)->pkt = (*it_r)->pkt;
      ++it_p;
      ++it_r;
    }
  }
}

void ForwardErrorCorrection::InsertPackets(
    ReceivedPacketList* received_packets,
    RecoveredPacketList* recovered_packets) {
  while (!received_packets->empty()) {
    ReceivedPacket* received_packet = received_packets->front().get();

    // Discard old FEC packets such that the sequence numbers in
    // |received_fec_packets_| span at most 1/2 of the sequence number space.
    // This is important for keeping |received_fec_packets_| sorted, and may
    // also reduce the possibility of incorrect decoding due to sequence number
    // wrap-around.
    // TODO(marpan/holmer): We should be able to improve detection/discarding of
    // old FEC packets based on timestamp information or better sequence number
    // thresholding (e.g., to distinguish between wrap-around and reordering).
    if (!received_fec_packets_.empty() &&
        received_packet->ssrc == received_fec_packets_.front()->ssrc) {
      // It only makes sense to detect wrap-around when |received_packet|
      // and |front_received_fec_packet| belong to the same sequence number
      // space, i.e., the same SSRC. This happens when |received_packet|
      // is a FEC packet, or if |received_packet| is a media packet and
      // RED+ULPFEC is used.
      auto it = received_fec_packets_.begin();
      while (it != received_fec_packets_.end()) {
        uint16_t seq_num_diff = abs(static_cast<int>(received_packet->seq_num) -
                                    static_cast<int>((*it)->seq_num));
        if (seq_num_diff > 0x3fff) {
          it = received_fec_packets_.erase(it);
        } else {
          // No need to keep iterating, since |received_fec_packets_| is sorted.
          break;
        }
      }
    }

    if (received_packet->is_fec) {
      InsertFecPacket(*recovered_packets, received_packet);
    } else {
      InsertMediaPacket(recovered_packets, received_packet);
    }
    // Delete the received packet "wrapper".
    received_packets->pop_front();
  }
  RTC_DCHECK(received_packets->empty());
  DiscardOldRecoveredPackets(recovered_packets);
}

bool ForwardErrorCorrection::StartPacketRecovery(
    const ReceivedFecPacket& fec_packet,
    RecoveredPacket* recovered_packet) {
  // Sanity check packet length.
  if (fec_packet.pkt->length < fec_packet.fec_header_size) {
    LOG(LS_WARNING)
        << "The FEC packet is truncated: it does not contain enough room "
        << "for its own header.";
    return false;
  }
  // Initialize recovered packet data.
  recovered_packet->pkt = new Packet();
  memset(recovered_packet->pkt->data, 0, IP_PACKET_SIZE);
  recovered_packet->returned = false;
  recovered_packet->was_recovered = true;
  // Copy bytes corresponding to minimum RTP header size.
  // Note that the sequence number and SSRC fields will be overwritten
  // at the end of packet recovery.
  memcpy(&recovered_packet->pkt->data, fec_packet.pkt->data, kRtpHeaderSize);
  // Copy remaining FEC payload.
  if (fec_packet.protection_length >
      std::min(sizeof(recovered_packet->pkt->data) - kRtpHeaderSize,
               sizeof(fec_packet.pkt->data) - fec_packet.fec_header_size)) {
    LOG(LS_WARNING) << "Incorrect protection length, dropping FEC packet.";
    return false;
  }
  memcpy(&recovered_packet->pkt->data[kRtpHeaderSize],
         &fec_packet.pkt->data[fec_packet.fec_header_size],
         fec_packet.protection_length);
  return true;
}

bool ForwardErrorCorrection::FinishPacketRecovery(
    const ReceivedFecPacket& fec_packet,
    RecoveredPacket* recovered_packet) {
  // Set the RTP version to 2.
  recovered_packet->pkt->data[0] |= 0x80;  // Set the 1st bit.
  recovered_packet->pkt->data[0] &= 0xbf;  // Clear the 2nd bit.
  // Recover the packet length, from temporary location.
  recovered_packet->pkt->length =
      ByteReader<uint16_t>::ReadBigEndian(&recovered_packet->pkt->data[2]) +
      kRtpHeaderSize;
  if (recovered_packet->pkt->length >
      sizeof(recovered_packet->pkt->data) - kRtpHeaderSize) {
    LOG(LS_WARNING) << "The recovered packet had a length larger than a "
                    << "typical IP packet, and is thus dropped.";
    return false;
  }
  // Set the SN field.
  ByteWriter<uint16_t>::WriteBigEndian(&recovered_packet->pkt->data[2],
                                       recovered_packet->seq_num);
  // Set the SSRC field.
  ByteWriter<uint32_t>::WriteBigEndian(&recovered_packet->pkt->data[8],
                                       fec_packet.protected_ssrc);
  recovered_packet->ssrc = fec_packet.protected_ssrc;
  return true;
}

void ForwardErrorCorrection::XorHeaders(const Packet& src, Packet* dst) {
  // XOR the first 2 bytes of the header: V, P, X, CC, M, PT fields.
  dst->data[0] ^= src.data[0];
  dst->data[1] ^= src.data[1];

  // XOR the length recovery field.
  uint8_t src_payload_length_network_order[2];
  ByteWriter<uint16_t>::WriteBigEndian(src_payload_length_network_order,
                                       src.length - kRtpHeaderSize);
  dst->data[2] ^= src_payload_length_network_order[0];
  dst->data[3] ^= src_payload_length_network_order[1];

  // XOR the 5th to 8th bytes of the header: the timestamp field.
  dst->data[4] ^= src.data[4];
  dst->data[5] ^= src.data[5];
  dst->data[6] ^= src.data[6];
  dst->data[7] ^= src.data[7];

  // Skip the 9th to 12th bytes of the header.
}

void ForwardErrorCorrection::XorPayloads(const Packet& src,
                                         size_t payload_length,
                                         size_t dst_offset,
                                         Packet* dst) {
  // XOR the payload.
  RTC_DCHECK_LE(kRtpHeaderSize + payload_length, sizeof(src.data));
  RTC_DCHECK_LE(dst_offset + payload_length, sizeof(dst->data));
  for (size_t i = 0; i < payload_length; ++i) {
    dst->data[dst_offset + i] ^= src.data[kRtpHeaderSize + i];
  }
}

bool ForwardErrorCorrection::RecoverPacket(const ReceivedFecPacket& fec_packet,
                                           RecoveredPacket* recovered_packet) {
  if (!StartPacketRecovery(fec_packet, recovered_packet)) {
    return false;
  }
  for (const auto& protected_packet : fec_packet.protected_packets) {
    if (protected_packet->pkt == nullptr) {
      // This is the packet we're recovering.
      recovered_packet->seq_num = protected_packet->seq_num;
    } else {
      XorHeaders(*protected_packet->pkt, recovered_packet->pkt);
      XorPayloads(*protected_packet->pkt, protected_packet->pkt->length,
                  kRtpHeaderSize, recovered_packet->pkt);
    }
  }
  if (!FinishPacketRecovery(fec_packet, recovered_packet)) {
    return false;
  }
  return true;
}

void ForwardErrorCorrection::AttemptRecovery(
    RecoveredPacketList* recovered_packets) {
  auto fec_packet_it = received_fec_packets_.begin();
  while (fec_packet_it != received_fec_packets_.end()) {
    // Search for each FEC packet's protected media packets.
    int packets_missing = NumCoveredPacketsMissing(**fec_packet_it);

    // We can only recover one packet with an FEC packet.
    if (packets_missing == 1) {
      // Recovery possible.
      std::unique_ptr<RecoveredPacket> recovered_packet(new RecoveredPacket());
      recovered_packet->pkt = nullptr;
      if (!RecoverPacket(**fec_packet_it, recovered_packet.get())) {
        // Can't recover using this packet, drop it.
        fec_packet_it = received_fec_packets_.erase(fec_packet_it);
        continue;
      }

      auto recovered_packet_ptr = recovered_packet.get();
      // Add recovered packet to the list of recovered packets and update any
      // FEC packets covering this packet with a pointer to the data.
      // TODO(holmer): Consider replacing this with a binary search for the
      // right position, and then just insert the new packet. Would get rid of
      // the sort.
      recovered_packets->push_back(std::move(recovered_packet));
      recovered_packets->sort(SortablePacket::LessThan());
      UpdateCoveringFecPackets(*recovered_packet_ptr);
      DiscardOldRecoveredPackets(recovered_packets);
      fec_packet_it = received_fec_packets_.erase(fec_packet_it);

      // A packet has been recovered. We need to check the FEC list again, as
      // this may allow additional packets to be recovered.
      // Restart for first FEC packet.
      fec_packet_it = received_fec_packets_.begin();
    } else if (packets_missing == 0) {
      // Either all protected packets arrived or have been recovered. We can
      // discard this FEC packet.
      fec_packet_it = received_fec_packets_.erase(fec_packet_it);
    } else {
      fec_packet_it++;
    }
  }
}

int ForwardErrorCorrection::NumCoveredPacketsMissing(
    const ReceivedFecPacket& fec_packet) {
  int packets_missing = 0;
  for (const auto& protected_packet : fec_packet.protected_packets) {
    if (protected_packet->pkt == nullptr) {
      ++packets_missing;
      if (packets_missing > 1) {
        break;  // We can't recover more than one packet.
      }
    }
  }
  return packets_missing;
}

void ForwardErrorCorrection::DiscardOldRecoveredPackets(
    RecoveredPacketList* recovered_packets) {
  const size_t max_media_packets = fec_header_reader_->MaxMediaPackets();
  while (recovered_packets->size() > max_media_packets) {
    recovered_packets->pop_front();
  }
  RTC_DCHECK_LE(recovered_packets->size(), max_media_packets);
}

uint16_t ForwardErrorCorrection::ParseSequenceNumber(uint8_t* packet) {
  return (packet[2] << 8) + packet[3];
}

uint32_t ForwardErrorCorrection::ParseSsrc(uint8_t* packet) {
  return (packet[8] << 24) + (packet[9] << 16) + (packet[10] << 8) + packet[11];
}

int ForwardErrorCorrection::DecodeFec(
    ReceivedPacketList* received_packets,
    RecoveredPacketList* recovered_packets) {
  RTC_DCHECK(received_packets);
  RTC_DCHECK(recovered_packets);

  const size_t max_media_packets = fec_header_reader_->MaxMediaPackets();
  if (recovered_packets->size() == max_media_packets) {
    const RecoveredPacket* back_recovered_packet =
        recovered_packets->back().get();
    for (const auto& received_packet : *received_packets) {
      if (received_packet->ssrc == back_recovered_packet->ssrc) {
        const unsigned int seq_num_diff =
            abs(static_cast<int>(received_packet->seq_num) -
                static_cast<int>(back_recovered_packet->seq_num));
        if (seq_num_diff > max_media_packets) {
          // A big gap in sequence numbers. The old recovered packets
          // are now useless, so it's safe to do a reset.
          LOG(LS_INFO) << "Big gap in media/ULPFEC sequence numbers. No need "
                          "to keep the old packets in the FEC buffers, thus "
                          "resetting them.";
          ResetState(recovered_packets);
          break;
        }
      }
    }
  }

  InsertPackets(received_packets, recovered_packets);
  AttemptRecovery(recovered_packets);

  return 0;
}

size_t ForwardErrorCorrection::MaxPacketOverhead() const {
  return fec_header_writer_->MaxPacketOverhead();
}

FecHeaderReader::FecHeaderReader(size_t max_media_packets,
                                 size_t max_fec_packets)
    : max_media_packets_(max_media_packets),
      max_fec_packets_(max_fec_packets) {}

FecHeaderReader::~FecHeaderReader() = default;

size_t FecHeaderReader::MaxMediaPackets() const {
  return max_media_packets_;
}

size_t FecHeaderReader::MaxFecPackets() const {
  return max_fec_packets_;
}

FecHeaderWriter::FecHeaderWriter(size_t max_media_packets,
                                 size_t max_fec_packets,
                                 size_t max_packet_overhead)
    : max_media_packets_(max_media_packets),
      max_fec_packets_(max_fec_packets),
      max_packet_overhead_(max_packet_overhead) {}

FecHeaderWriter::~FecHeaderWriter() = default;

size_t FecHeaderWriter::MaxMediaPackets() const {
  return max_media_packets_;
}

size_t FecHeaderWriter::MaxFecPackets() const {
  return max_fec_packets_;
}

size_t FecHeaderWriter::MaxPacketOverhead() const {
  return max_packet_overhead_;
}

}  // namespace webrtc