Generalize FEC header formatting. (pt. 4)

webrtc/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/base/checks.h"
 #include "webrtc/base/logging.h"
 #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/forward_error_correction_internal.h"
+#include "webrtc/modules/rtp_rtcp/source/ulpfec_header_reader_writer.h"
 
 namespace webrtc {
 
-// FEC header size in bytes.
-constexpr size_t kFecHeaderSize = 10;
-
-// ULP header size in bytes (L bit is set).
-constexpr size_t kUlpHeaderSizeLBitSet = (2 + kMaskSizeLBitSet);
-
-// ULP header size in bytes (L bit is cleared).
-constexpr size_t kUlpHeaderSizeLBitClear = (2 + kMaskSizeLBitClear);
-
+namespace {
 // Transport header size in bytes. Assume UDP/IPv4 as a reasonable minimum.
 constexpr size_t kTransportOverhead = 28;
+}  // namespace
 
-// Maximum number of media packets that can be protected.
-constexpr size_t ForwardErrorCorrection::kMaxMediaPackets;
-
-// Maximum number of FEC packets stored internally.
-constexpr size_t kMaxFecPackets = ForwardErrorCorrection::kMaxMediaPackets;
+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) {
   return IsNewerSequenceNumber(second->seq_num, first->seq_num);
 }
 
-ForwardErrorCorrection::ReceivedPacket::ReceivedPacket() {}
-ForwardErrorCorrection::ReceivedPacket::~ReceivedPacket() {}
+ForwardErrorCorrection::ReceivedPacket::ReceivedPacket() = default;
+ForwardErrorCorrection::ReceivedPacket::~ReceivedPacket() = default;
 
-ForwardErrorCorrection::RecoveredPacket::RecoveredPacket() {}
-ForwardErrorCorrection::RecoveredPacket::~RecoveredPacket() {}
+ForwardErrorCorrection::RecoveredPacket::RecoveredPacket() = default;
+ForwardErrorCorrection::RecoveredPacket::~RecoveredPacket() = default;
 
-ForwardErrorCorrection::ForwardErrorCorrection()
-    : generated_fec_packets_(kMaxMediaPackets), received_fec_packets_(),
-      packet_mask_(), tmp_packet_mask_() {}
-ForwardErrorCorrection::~ForwardErrorCorrection() {}
+ForwardErrorCorrection::ProtectedPacket::ProtectedPacket() = default;
+ForwardErrorCorrection::ProtectedPacket::~ProtectedPacket() = default;
 
-// Input packet
-//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-//   |                    RTP Header (12 octets)                     |
-//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-//   |                         RTP Payload                           |
-//   |                                                               |
-//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ForwardErrorCorrection::ReceivedFecPacket::ReceivedFecPacket() = default;
+ForwardErrorCorrection::ReceivedFecPacket::~ReceivedFecPacket() = default;
 
-// Output packet
-//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-//   |                    FEC Header (10 octets)                     |
-//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-//   |                      FEC Level 0 Header                       |
-//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-//   |                     FEC Level 0 Payload                       |
-//   |                                                               |
-//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-//
+ForwardErrorCorrection::ForwardErrorCorrection(
+    std::unique_ptr<FecHeaderReader> fec_header_reader,
+    std::unique_ptr<FecHeaderWriter> fec_header_writer)
+    : 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) {}
+
+std::unique_ptr<ForwardErrorCorrection> ForwardErrorCorrection::CreateUlpfec() {
+  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)));
+}
+
 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, 0u);
   RTC_DCHECK_GE(num_important_packets, 0);
   RTC_DCHECK_LE(static_cast<size_t>(num_important_packets), num_media_packets);
   RTC_DCHECK(fec_packets->empty());
-
-  if (num_media_packets > kMaxMediaPackets) {
+  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 " << kMaxMediaPackets
+                    << " media packets per frame. Max is " << max_media_packets
                     << ".";
     return -1;
   }
 
-  bool l_bit = (num_media_packets > 8 * kMaskSizeLBitClear);
-  int num_mask_bytes = l_bit ? kMaskSizeLBitSet : kMaskSizeLBitClear;
-
   // 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;
   }
-
-  // Prepare generated FEC packets by setting them to 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);
-
-  // -- Generate packet masks --
-  memset(packet_mask_, 0, num_fec_packets * num_mask_bytes);
+  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_mask_);
+                                mask_table, packet_masks_);
 
-  int num_mask_bits = InsertZerosInBitMasks(
-      media_packets, packet_mask_, num_mask_bytes, num_fec_packets);
-
+  // Adapt packet masks to missing media packets.
+  int num_mask_bits = InsertZerosInPacketMasks(media_packets, num_fec_packets);
   if (num_mask_bits < 0) {
     return -1;
   }
-  l_bit = (static_cast<size_t>(num_mask_bits) > 8 * kMaskSizeLBitClear);
-  if (l_bit) {
-    num_mask_bytes = kMaskSizeLBitSet;
-  }
+  packet_mask_size_ = internal::PacketMaskSize(num_mask_bits);
 
-  GenerateFecBitStrings(media_packets, packet_mask_, num_fec_packets, l_bit);
-  GenerateFecUlpHeaders(media_packets, packet_mask_, num_fec_packets, l_bit);
+  // Write FEC packets to |generated_fec_packets_|.
+  GenerateFecPayloads(media_packets, num_fec_packets);
+  FinalizeFecHeaders(media_packets, num_fec_packets);
 
   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::GenerateFecBitStrings(
+void ForwardErrorCorrection::GenerateFecPayloads(
     const PacketList& media_packets,
-    uint8_t* packet_mask,
-    int num_fec_packets,
-    bool l_bit) {
+    size_t num_fec_packets) {
   RTC_DCHECK(!media_packets.empty());
-  uint8_t media_payload_length[2];
-  const int num_mask_bytes = l_bit ? kMaskSizeLBitSet : kMaskSizeLBitClear;
-  const uint16_t ulp_header_size =
-      l_bit ? kUlpHeaderSizeLBitSet : kUlpHeaderSizeLBitClear;
-  const uint16_t fec_rtp_offset =
-      kFecHeaderSize + ulp_header_size - kRtpHeaderSize;
+  for (size_t i = 0; i < num_fec_packets; ++i) {
+    Packet* const fec_packet = &generated_fec_packets_[i];
+    uint32_t pkt_mask_idx = i * packet_mask_size_;

[danilchap, 2016/08/29 12:26:02]

since you touch almost all lines in this function, change this type to size_t
too.

[brandtr, 2016/08/30 07:12:40]

Done.

+    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);
 
-  for (int i = 0; i < num_fec_packets; ++i) {
-    Packet* const fec_packet = &generated_fec_packets_[i];
+    uint32_t media_pkt_idx = 0;

[danilchap, 2016/08/29 12:26:02]

size_t

[brandtr, 2016/08/30 07:12:40]

Done.

     auto media_packets_it = media_packets.cbegin();
-    uint32_t pkt_mask_idx = i * num_mask_bytes;
-    uint32_t media_pkt_idx = 0;
-    uint16_t fec_packet_length = 0;
     uint16_t prev_seq_num = ParseSequenceNumber((*media_packets_it)->data);
     while (media_packets_it != media_packets.end()) {
-      // Each FEC packet has a multiple byte mask. Determine if this media
-      // packet should be included in FEC packet i.
-      if (packet_mask[pkt_mask_idx] & (1 << (7 - media_pkt_idx))) {
-        Packet* media_packet = media_packets_it->get();
+      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;
+        uint8_t media_payload_length_network_order[2];
+        ByteWriter<uint16_t>::WriteBigEndian(media_payload_length_network_order,

[danilchap, 2016/08/29 12:26:02]

this now used in if (first_protected_packet) only.
May be move it here and WriteBigEndian directly into &fec_packet->data[2]

[brandtr, 2016/08/30 07:12:40]

Done.

+                                             media_payload_length);
 
-        // Assign network-ordered media payload length.
-        ByteWriter<uint16_t>::WriteBigEndian(
-            media_payload_length, media_packet->length - kRtpHeaderSize);
-
-        fec_packet_length = media_packet->length + fec_rtp_offset;
-        // On the first protected packet, we don't need to XOR.
-        if (fec_packet->length == 0) {
-          // Copy the first 2 bytes of the RTP header. Note that the E and L
-          // bits are overwritten in GenerateFecUlpHeaders.
+        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);
-          // Copy the 5th to 8th bytes of the RTP header (timestamp).
+          // Write length recovery field. (This is a temporary location for
+          // ULPFEC.)
+          memcpy(&fec_packet->data[2], media_payload_length_network_order, 2);
+          // Write timestamp recovery field.
           memcpy(&fec_packet->data[4], &media_packet->data[4], 4);
-          // Copy network-ordered payload size.
-          memcpy(&fec_packet->data[8], media_payload_length, 2);
-
-          // Copy RTP payload, leaving room for the ULP header.
-          memcpy(&fec_packet->data[kFecHeaderSize + ulp_header_size],
-                 &media_packet->data[kRtpHeaderSize],
-                 media_packet->length - kRtpHeaderSize);
+          // Write payload.
+          memcpy(&fec_packet->data[fec_header_size],
+                 &media_packet->data[kRtpHeaderSize], media_payload_length);
         } else {
-          // XOR with the first 2 bytes of the RTP header.
-          fec_packet->data[0] ^= media_packet->data[0];
-          fec_packet->data[1] ^= media_packet->data[1];
-
-          // XOR with the 5th to 8th bytes of the RTP header.
-          for (uint32_t j = 4; j < 8; ++j) {
-            fec_packet->data[j] ^= media_packet->data[j];
-          }
-
-          // XOR with the network-ordered payload size.
-          fec_packet->data[8] ^= media_payload_length[0];
-          fec_packet->data[9] ^= media_payload_length[1];
-
-          // XOR with RTP payload, leaving room for the ULP header.
-          for (int32_t j = kFecHeaderSize + ulp_header_size;
-               j < fec_packet_length; j++) {
-            fec_packet->data[j] ^= media_packet->data[j - fec_rtp_offset];
-          }
-        }
-        if (fec_packet_length > fec_packet->length) {
-          fec_packet->length = fec_packet_length;
+          XorPackets(*media_packet, kRtpHeaderSize,
+                     fec_packet_length - fec_header_size, fec_header_size,

[danilchap, 2016/08/29 12:26:02]

maybe media_payload_length instead of fec_packet_length - fec_header_size
to make more consistent with if (first_protected_packet) block

[brandtr, 2016/08/30 07:12:40]

Done.

+                     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, 0u)
         << "Packet mask is wrong or poorly designed.";
   }
 }
 
-int ForwardErrorCorrection::InsertZerosInBitMasks(
+int ForwardErrorCorrection::InsertZerosInPacketMasks(
     const PacketList& media_packets,
-    uint8_t* packet_mask,
-    int num_mask_bytes,
-    int num_fec_packets) {
-  if (media_packets.size() <= 1) {
-    return media_packets.size();
+    size_t num_fec_packets) {
+  size_t num_media_packets = media_packets.size();
+  if (num_media_packets <= 1) {
+    return num_media_packets;
   }
-  int last_seq_num = ParseSequenceNumber(media_packets.back()->data);
-  int first_seq_num = ParseSequenceNumber(media_packets.front()->data);
-  int total_missing_seq_nums =
-      static_cast<uint16_t>(last_seq_num - first_seq_num) -
-      media_packets.size() + 1;
+  uint16_t last_seq_num = ParseSequenceNumber(media_packets.back()->data);
+  uint16_t first_seq_num = ParseSequenceNumber(media_packets.front()->data);
+  uint16_t total_missing_seq_nums =

[danilchap, 2016/08/29 12:26:02]

this variable represent number of, not a sequence number, so make it size_t.

[brandtr, 2016/08/30 07:12:40]

Done.

+      static_cast<uint16_t>(last_seq_num - first_seq_num) - num_media_packets +

[danilchap, 2016/08/29 12:26:02]

this static_cast shouldn't be needed now:
uint16_t - uint16_t = uint16_t.

[brandtr, 2016/08/30 07:12:40]

Seems to still be needed for correct wrapping, after change of
|total_missing_seq_nums| to size_t.

[danilchap, 2016/08/30 12:29:01]

Yep, I was wrong, better leave this static_cast.
Not sure why it is needed from compiler point of view.
So to avoid confusing humans better to leave explicit cast to stress wrapping
support.

[brandtr, 2016/08/30 14:14:24]

I guess it has something to do with promotion logic during arithmetic? It
compiles fine without the cast, but then some of the tests that check wrapping
behaviour fail.

+      1;
   if (total_missing_seq_nums == 0) {
-    // All sequence numbers are covered by the packet mask. No zero insertion
-    // required.
-    return media_packets.size();
+    // All sequence numbers are covered by the packet mask.
+    // No zero insertion required.
+    return num_media_packets;
   }
-  // We can only protect 8 * kMaskSizeLBitSet packets.
-  if (total_missing_seq_nums + media_packets.size() > 8 * kMaskSizeLBitSet)
+  if (total_missing_seq_nums + num_media_packets > kUlpfecMaxMediaPackets) {

[danilchap, 2016/08/29 12:26:02]

probably fec_header_writer_->MaxMediaPackets() instead of kUlpfecMaxMediaPackets

[brandtr, 2016/08/30 07:12:40]

Done.

     return -1;
+  }
   // Allocate the new mask.
-  int new_mask_bytes = kMaskSizeLBitClear;
-  if (media_packets.size() +
-      total_missing_seq_nums > 8 * kMaskSizeLBitClear) {
-    new_mask_bytes = kMaskSizeLBitSet;
-  }
-  memset(tmp_packet_mask_, 0, num_fec_packets * kMaskSizeLBitSet);
+  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_mask_, new_mask_bytes, packet_mask_,
-                       num_mask_bytes, num_fec_packets, 0, 0);
+  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 == 8 * kMaskSizeLBitSet) {
+    if (new_bit_index == kUlpfecMaxMediaPackets) {

[danilchap, 2016/08/29 12:26:02]

ditto

[brandtr, 2016/08/30 07:12:40]

Done.

       // 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_mask_,
-                                  new_mask_bytes, num_fec_packets,
+      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_mask_, new_mask_bytes, packet_mask_,
-                         num_mask_bytes, num_fec_packets, new_bit_index,
+    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 * new_mask_bytes + new_bit_index / 8;
-      tmp_packet_mask_[new_byte_index] <<= (7 - (new_bit_index % 8));
+      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_mask, tmp_packet_mask_, kMaskSizeLBitSet * num_fec_packets);
+  memcpy(packet_masks_, tmp_packet_masks_,
+         num_fec_packets * tmp_packet_mask_size);
   return new_bit_index;
 }
 
-void ForwardErrorCorrection::GenerateFecUlpHeaders(
-    const PacketList& media_packets,
-    uint8_t* packet_mask,
-    int num_fec_packets,
-    bool l_bit) {
-  // -- Generate FEC and ULP headers --
-  //
-  // FEC Header, 10 bytes
-  //    0                   1                   2                   3
-  //    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
-  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-  //   |E|L|P|X|  CC   |M| PT recovery |            SN base            |
-  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-  //   |                          TS recovery                          |
-  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-  //   |        length recovery        |
-  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-  //
-  // ULP Header, 4 bytes (for L = 0)
-  //    0                   1                   2                   3
-  //    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
-  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-  //   |       Protection Length       |             mask              |
-  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-  //   |              mask cont. (present only when L = 1)             |
-  //   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-  int num_mask_bytes = l_bit ? kMaskSizeLBitSet : kMaskSizeLBitClear;
-  const uint16_t ulp_header_size =
-      l_bit ? kUlpHeaderSizeLBitSet : kUlpHeaderSizeLBitClear;
-
-  RTC_DCHECK(!media_packets.empty());
-  Packet* first_media_packet = media_packets.front().get();
-  RTC_DCHECK(first_media_packet);
-  uint16_t seq_num = ParseSequenceNumber(first_media_packet->data);
-  for (int i = 0; i < num_fec_packets; ++i) {
-    Packet* const fec_packet = &generated_fec_packets_[i];
-    // -- FEC header --
-    fec_packet->data[0] &= 0x7f;  // Set E to zero.
-    if (l_bit == 0) {
-      fec_packet->data[0] &= 0xbf;  // Clear the L bit.
-    } else {
-      fec_packet->data[0] |= 0x40;  // Set the L bit.
-    }
-    // Sequence number from first media packet used as SN base.
-    // We use the same sequence number base for every FEC packet,
-    // but that's not required in general.
-    ByteWriter<uint16_t>::WriteBigEndian(&fec_packet->data[2], seq_num);
-
-    // -- ULP header --
-    // Copy the payload size to the protection length field.
-    // (We protect the entire packet.)
-    ByteWriter<uint16_t>::WriteBigEndian(
-        &fec_packet->data[10],
-        fec_packet->length - kFecHeaderSize - ulp_header_size);
-
-    // Copy the packet mask.
-    memcpy(&fec_packet->data[12], &packet_mask[i * num_mask_bytes],
-           num_mask_bytes);
+void ForwardErrorCorrection::FinalizeFecHeaders(const PacketList& media_packets,
+                                                size_t num_fec_packets) {
+  for (size_t i = 0; i < num_fec_packets; ++i) {
+    fec_header_writer_->FinalizeFecHeader(
+        media_packets, &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(
-    ReceivedPacket* received_packet,
-    RecoveredPacketList* recovered_packets) {
-
+    RecoveredPacketList* recovered_packets,
+    ReceivedPacket* received_packet) {
   // Search for duplicate packets.
   for (const auto& recovered_packet : *recovered_packets) {
     if (received_packet->seq_num == recovered_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->seq_num = received_packet->seq_num;
   recovered_packet->pkt = received_packet->pkt;
   recovered_packet->pkt->length = received_packet->pkt->length;
-
-  RecoveredPacket* recovered_packet_ptr = recovered_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.
+  RecoveredPacket* recovered_packet_ptr = recovered_packet.get();
   recovered_packets->push_back(std::move(recovered_packet));
   recovered_packets->sort(SortablePacket::LessThan());
-  UpdateCoveringFecPackets(recovered_packet_ptr);
+  UpdateCoveringFecPackets(*recovered_packet_ptr);
 }
 
-void ForwardErrorCorrection::UpdateCoveringFecPackets(RecoveredPacket* packet) {
+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());
+                                         &packet, SortablePacket::LessThan());
     if (protected_it != fec_packet->protected_packets.end() &&
-        (*protected_it)->seq_num == packet->seq_num) {
+        (*protected_it)->seq_num == packet.seq_num) {
       // Found an FEC packet which is protecting |packet|.
-      (*protected_it)->pkt = packet->pkt;
+      (*protected_it)->pkt = packet.pkt;
     }
   }
 }
 
 void ForwardErrorCorrection::InsertFecPacket(
-    ReceivedPacket* received_packet,
-    const RecoveredPacketList* recovered_packets) {
+    const RecoveredPacketList& recovered_packets,
+    ReceivedPacket* received_packet) {
   // Check for duplicate.
   for (const auto& existing_fec_packet : received_fec_packets_) {
     if (received_packet->seq_num == existing_fec_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->seq_num = received_packet->seq_num;
-  fec_packet->ssrc = received_packet->ssrc;
-
-  const uint16_t seq_num_base =
-      ByteReader<uint16_t>::ReadBigEndian(&fec_packet->pkt->data[2]);
-  const uint16_t mask_size_bytes = (fec_packet->pkt->data[0] & 0x40)
-                                      ? kMaskSizeLBitSet
-                                      : kMaskSizeLBitClear;  // L bit set?
-
-  // Parse erasure code mask from ULP header and represent as protected packets.
-  for (uint16_t byte_idx = 0; byte_idx < mask_size_bytes; ++byte_idx) {
-    uint8_t packet_mask = fec_packet->pkt->data[12 + 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->seq_num =
-            static_cast<uint16_t>(seq_num_base + (byte_idx << 3) + bit_idx);
-        protected_packet->pkt = nullptr;
-        // Note that |protected_pkt_list| is sorted (according to sequence
-        // number) by construction.
-        fec_packet->protected_packets.push_back(std::move(protected_packet));
+  fec_packet->rtp_ssrc = received_packet->ssrc;
+  // Parse ULPFEC/FlexFEC header specific info.
+  bool ret = fec_header_reader_->ReadFecHeader(fec_packet.get());
+  if (!ret) {
+    LOG(LS_WARNING) << "Malformed FEC header: dropping packet.";
+    return;
+  }
+  // Parse packet mask from header and represent as protected packets.
+  for (const auto& stream_packet_mask_info : fec_packet->packet_mask_infos) {
+    uint16_t seq_num_base = stream_packet_mask_info.second.seq_num_base;
+    size_t offset = stream_packet_mask_info.second.offset;
+    size_t size = stream_packet_mask_info.second.size;
+    for (uint16_t byte_idx = 0; byte_idx < size; ++byte_idx) {
+      uint8_t packet_mask = fec_packet->pkt->data[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->seq_num =
+              static_cast<uint16_t>(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(fec_packet.get(), recovered_packets);
+    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.
     //
-    // For correct decoding, |fec_packet_list_| does not necessarily
+    // For correct decoding, |received_fec_packets_| does not necessarily
     // need to be sorted by sequence number (see decoding algorithm in
-    // AttemptRecover()), but by keeping it sorted we try to recover the
-    // oldest lost packets first.
+    // AttemptRecover()). By keeping it sorted we try to recover the
+    // oldest lost packets first, however.
     received_fec_packets_.push_back(std::move(fec_packet));
     received_fec_packets_.sort(SortablePacket::LessThan());
-    if (received_fec_packets_.size() > kMaxFecPackets) {
+    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(), kMaxFecPackets);
+    RTC_DCHECK_LE(received_fec_packets_.size(), max_fec_packets);
   }
 }
 
 void ForwardErrorCorrection::AssignRecoveredPackets(
-    ReceivedFecPacket* fec_packet,
-    const RecoveredPacketList* recovered_packets) {
+    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();
+  auto it_r = recovered_packets.cbegin();
   SortablePacket::LessThan less_than;
-  while (it_p != protected_packets->end() && it_r != recovered_packets->end()) {
+  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;
     }
@@ skipping 16 matching lines @@
     if (!received_fec_packets_.empty()) {
       uint16_t seq_num_diff =
           abs(static_cast<int>(received_packet->seq_num) -
               static_cast<int>(received_fec_packets_.front()->seq_num));
       if (seq_num_diff > 0x3fff) {
         received_fec_packets_.pop_front();
       }
     }
 
     if (received_packet->is_fec) {
-      InsertFecPacket(received_packet, recovered_packets);
+      InsertFecPacket(*recovered_packets, received_packet);
     } else {
-      InsertMediaPacket(received_packet, recovered_packets);
+      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) {
-  // This is the first packet which we try to recover with.
-  const uint16_t ulp_header_size = fec_packet->pkt->data[0] & 0x40
-                                       ? kUlpHeaderSizeLBitSet
-                                       : kUlpHeaderSizeLBitClear;  // L bit set?
-  if (fec_packet->pkt->length <
-      static_cast<size_t>(kFecHeaderSize + ulp_header_size)) {
+    const ReceivedFecPacket& fec_packet,
+    RecoveredPacket* recovered_packet) {
+  // Sanity check packet length.
+  if (fec_packet.pkt->length < fec_packet.fec_header_size) {
     LOG(LS_WARNING)
-        << "Truncated FEC packet doesn't contain room for ULP header.";
+        << "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;
-  uint16_t protection_length =
-      ByteReader<uint16_t>::ReadBigEndian(&fec_packet->pkt->data[10]);
-  if (protection_length >
-      std::min(
-          sizeof(recovered_packet->pkt->data) - kRtpHeaderSize,
-          sizeof(fec_packet->pkt->data) - kFecHeaderSize - ulp_header_size)) {
-    LOG(LS_WARNING) << "Incorrect FEC protection length, dropping.";
+  // 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;
   }
-  // Copy FEC payload, skipping the ULP header.
   memcpy(&recovered_packet->pkt->data[kRtpHeaderSize],
-         &fec_packet->pkt->data[kFecHeaderSize + ulp_header_size],
-         protection_length);
-  // Copy the length recovery field.
-  memcpy(recovered_packet->length_recovery, &fec_packet->pkt->data[8], 2);
-  // Copy the first 2 bytes of the FEC header.
-  memcpy(recovered_packet->pkt->data, fec_packet->pkt->data, 2);
-  // Copy the 5th to 8th bytes of the FEC header.
-  memcpy(&recovered_packet->pkt->data[4], &fec_packet->pkt->data[4], 4);
-  // Set the SSRC field.
-  ByteWriter<uint32_t>::WriteBigEndian(&recovered_packet->pkt->data[8],
-                                       fec_packet->ssrc);
+         &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);
-  // Recover the packet length.
-  recovered_packet->pkt->length =
-      ByteReader<uint16_t>::ReadBigEndian(recovered_packet->length_recovery) +
-      kRtpHeaderSize;
-  if (recovered_packet->pkt->length >
-      sizeof(recovered_packet->pkt->data) - kRtpHeaderSize) {
-    return false;
-  }
-
+  // Set the SSRC field.
+  ByteWriter<uint32_t>::WriteBigEndian(&recovered_packet->pkt->data[8],
+                                       fec_packet.rtp_ssrc);
   return true;
 }
 
-void ForwardErrorCorrection::XorPackets(const Packet* src,
-                                        RecoveredPacket* dst) {
-  // XOR with the first 2 bytes of the RTP header.
-  for (uint32_t i = 0; i < 2; ++i) {
-    dst->pkt->data[i] ^= src->data[i];
-  }
-  // XOR with the 5th to 8th bytes of the RTP header.
-  for (uint32_t i = 4; i < 8; ++i) {
-    dst->pkt->data[i] ^= src->data[i];
-  }
-  // XOR with the network-ordered payload size.
-  uint8_t media_payload_length[2];
-  ByteWriter<uint16_t>::WriteBigEndian(media_payload_length,
-                                       src->length - kRtpHeaderSize);
-  dst->length_recovery[0] ^= media_payload_length[0];
-  dst->length_recovery[1] ^= media_payload_length[1];
+void ForwardErrorCorrection::XorPackets(const Packet& src,
+                                        size_t src_offset,
+                                        size_t payload_length,
+                                        size_t dst_offset,
+                                        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 with RTP payload.
-  // TODO(marpan/ajm): Are we doing more XORs than required here?
-  for (size_t i = kRtpHeaderSize; i < src->length; ++i) {
-    dst->pkt->data[i] ^= src->data[i];
+  // 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.
+
+  // XOR the payload.
+  RTC_DCHECK_LE(src_offset + payload_length, sizeof(src.data));

[danilchap, 2016/08/29 12:26:02]

may be src.length/dst->length instead of sizeof
to check you do not read/write beyound size instead of beyond capacity.

[brandtr, 2016/08/30 07:12:40]

This is a good idea, but I'll wait until a later CL to do that. (The CL when I
split ForwardErrorCorrection into two classes.) Currently we rely on the fact
that the packet payload is zeroed out in the elements between the size and the
capacity. This helps for the case when we don't know the size of the recovered
packet until recovery is finished. That is, we actually do a bit too many XORs
here sometimes.

The reason that I will wait fixing it is that the requirements might be a bit
different between the sending and receiving sides XORing, and there is not point
in writing a generalized operation here, if it will be dismantled in a later CL.

The current DCHECKS were added just to ensure that we are never out of bounds.

+  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[src_offset + i];
   }
 }
 
-bool ForwardErrorCorrection::RecoverPacket(
-    const ReceivedFecPacket* fec_packet,
-    RecoveredPacket* rec_packet_to_insert) {
-  if (!StartPacketRecovery(fec_packet, rec_packet_to_insert))
+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) {
+  }
+  for (const auto& protected_packet : fec_packet.protected_packets) {
     if (protected_packet->pkt == nullptr) {
       // This is the packet we're recovering.
-      rec_packet_to_insert->seq_num = protected_packet->seq_num;
+      recovered_packet->seq_num = protected_packet->seq_num;
     } else {
-      XorPackets(protected_packet->pkt, rec_packet_to_insert);
+      XorPackets(*protected_packet->pkt, kRtpHeaderSize,
+                 protected_packet->pkt->length, kRtpHeaderSize,
+                 recovered_packet->pkt);
     }
   }
-  if (!FinishPacketRecovery(rec_packet_to_insert))
+  if (!FinishPacketRecovery(fec_packet, recovered_packet)) {
     return false;
+  }
   return true;
 }
 
-void ForwardErrorCorrection::AttemptRecover(
+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->get());
+    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> packet_to_insert(new RecoveredPacket());
-      packet_to_insert->pkt = nullptr;
-      if (!RecoverPacket(fec_packet_it->get(), packet_to_insert.get())) {
+      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 packet_to_insert_ptr = packet_to_insert.get();
+      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(packet_to_insert));
+      recovered_packets->push_back(std::move(recovered_packet));
       recovered_packets->sort(SortablePacket::LessThan());
-      UpdateCoveringFecPackets(packet_to_insert_ptr);
+      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) {
+    const ReceivedFecPacket& fec_packet) {
   int packets_missing = 0;
-  for (const auto& protected_packet : fec_packet->protected_packets) {
+  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) {
-  while (recovered_packets->size() > kMaxMediaPackets) {
+  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(), kMaxMediaPackets);
+  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) {
   // TODO(marpan/ajm): can we check for multiple ULP headers, and return an
   // error?
-  if (recovered_packets->size() == kMaxMediaPackets) {
+  const size_t max_media_packets = fec_header_reader_->MaxMediaPackets();
+  if (recovered_packets->size() == max_media_packets) {
     const unsigned int seq_num_diff =
         abs(static_cast<int>(received_packets->front()->seq_num) -
             static_cast<int>(recovered_packets->back()->seq_num));
-    if (seq_num_diff > kMaxMediaPackets) {
+    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.
       ResetState(recovered_packets);
     }
   }
   InsertPackets(received_packets, recovered_packets);
-  AttemptRecover(recovered_packets);
+  AttemptRecovery(recovered_packets);
   return 0;
 }
 
 size_t ForwardErrorCorrection::MaxPacketOverhead() const {
-  return kFecHeaderSize + kUlpHeaderSizeLBitSet;
+  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