Add packetization and coding/decoding of feedback message format.

webrtc/modules/rtp_rtcp/source/rtcp_packet/feedback_packet.cc

+/*
+ *  Copyright (c) 2015 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/rtcp_packet/feedback_packet.h"
+
+#include "webrtc/base/checks.h"
+#include "webrtc/base/logging.h"
+#include "webrtc/modules/rtp_rtcp/source/byte_io.h"
+
+namespace webrtc {
+namespace rtcp {
+
+// Header size:
+// * 12 bytes Common Packet Format for RTCP Feedback Messages
+// * 8 bytes FeedbackPacket header
+static const int kHeaderSizeBytes = 12 + 8;
+static const int kChunkSizeBytes = 2;
+static const int kOneBitVectorCapacity = 14;
+static const int kTwoBitVectorCapacity = 7;
+static const int kRunLengthCapacity = 0x1FFF;
+// TODO(sprang): Add support for dynamic max size for easier fragmentation,
+// eg. set it to what's left in the buffer or IP_PACKET_SIZE.
+static const int kMaxSizeBytes = (1 << 16) * 4;

[stefan-webrtc, 2015/07/27 11:49:28]

Why is it set to this? Seems like a large packet?

[sprang_webrtc, 2015/07/28 08:28:54]

Max length of an RTCP packet, as specified by the common RTCP header.
16bit length in four byte words, excluding the first header word.

[stefan-webrtc, 2015/07/28 09:34:10]

Ah, comment please. :)

[sprang_webrtc, 2015/07/28 11:39:22]

Done.

+static const int kMinSizeBytes = kHeaderSizeBytes + kChunkSizeBytes;
+static const int kBaseScaleFactor =
+    FeedbackPacket::kDeltaScaleFactor * (1 << 8);
+
+class PacketStatusChunk {
+ public:
+  virtual ~PacketStatusChunk() {}
+  virtual uint16_t NumSymbols() const = 0;
+  virtual void AppendSymbolsTo(
+      std::vector<FeedbackPacket::StatusSymbol>* vec) const = 0;
+  virtual void WriteTo(uint8_t* buffer) const = 0;
+};
+
+uint8_t EncodeSymbol(FeedbackPacket::StatusSymbol symbol) {
+  switch (symbol) {
+    case FeedbackPacket::StatusSymbol::kNotReceived:
+      return 0;
+    case FeedbackPacket::StatusSymbol::kReceivedSmallDelta:
+      return 1;
+    case FeedbackPacket::StatusSymbol::kReceivedLargeDelta:
+      return 2;
+    default:
+      abort();
+  }
+}
+
+FeedbackPacket::StatusSymbol DecodeSymbol(uint8_t value) {
+  switch (value) {
+    case 0:
+      return FeedbackPacket::StatusSymbol::kNotReceived;
+    case 1:
+      return FeedbackPacket::StatusSymbol::kReceivedSmallDelta;
+    case 2:
+      return FeedbackPacket::StatusSymbol::kReceivedLargeDelta;
+    default:
+      RTC_NOTREACHED();
+      return FeedbackPacket::StatusSymbol::kNotReceived;
+  }
+}
+
+FeedbackPacket::FeedbackPacket()
+    : packet_sender_ssrc_(0),
+      media_source_ssrc_(0),
+      base_seq_(-1),
+      base_time_(-1),
+      feedback_seq_(0),
+      last_seq_(-1),
+      last_timestamp_(-1),
+      first_symbol_cardinality_(0),
+      vec_needs_two_bit_symbols_(false),
+      size_bytes_(kHeaderSizeBytes) {
+}
+
+FeedbackPacket::~FeedbackPacket() {
+  for (PacketStatusChunk* chunk : status_chunks_)
+    delete chunk;
+}
+
+//  One Bit Status Vector Chunk
+//
+//  0                   1
+//  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//  |T|S|       symbol list         |
+//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//
+//  T = 1
+//  S = 0
+//  symbol list = 14 entries where 0 = not received, 1 = received
+
+class OneBitVectorChunk : public PacketStatusChunk {
+ public:
+  static const int kCapacity = 14;
+
+  explicit OneBitVectorChunk(
+      std::deque<FeedbackPacket::StatusSymbol>* symbols) {
+    size_t input_size = symbols->size();
+    for (size_t i = 0; i < kCapacity; ++i) {
+      if (i < input_size) {
+        symbols_[i] = symbols->front();
+        symbols->pop_front();
+      } else {
+        symbols_[i] = FeedbackPacket::StatusSymbol::kNotReceived;

[stefan-webrtc, 2015/07/27 11:49:28]

Hm, how do we handle the case if there are fewer symbols than 14? Do we mark
future packets as not received?

-- Nevermind, I see now that we can know how many we should ignore when parsing.

[sprang_webrtc, 2015/07/28 08:28:54]

Acknowledged.

+      }
+    }
+  }
+
+  virtual ~OneBitVectorChunk() {}
+
+  uint16_t NumSymbols() const override { return kCapacity; }
+
+  void AppendSymbolsTo(
+      std::vector<FeedbackPacket::StatusSymbol>* vec) const override {
+    vec->insert(vec->end(), &symbols_[0], &symbols_[kCapacity]);
+  }
+
+  void WriteTo(uint8_t* buffer) const override {
+    buffer[0] = 0x80;
+    buffer[0] |= EncodeSymbol(symbols_[0]) << 5;

[stefan-webrtc, 2015/07/27 11:49:28]

Should we DCHECK that symbols_[i] < 2?

[sprang_webrtc, 2015/07/28 08:28:54]

That should be caught by EncodeSymbol, no?

[stefan-webrtc, 2015/07/28 09:34:10]

Hm, maybe I misunderstood it then. Doesn't this OneBitVectorChunk encode one bit
per symbol? EncodeSymbol could potentially return 2 bits.

[sprang_webrtc, 2015/07/28 11:39:22]

Or maybe you have thought this through more than me :)
Added new check.

+    buffer[0] |= EncodeSymbol(symbols_[1]) << 4;
+    buffer[0] |= EncodeSymbol(symbols_[2]) << 3;
+    buffer[0] |= EncodeSymbol(symbols_[3]) << 2;
+    buffer[0] |= EncodeSymbol(symbols_[4]) << 1;
+    buffer[0] |= EncodeSymbol(symbols_[5]);
+    buffer[1] = EncodeSymbol(symbols_[6]) << 7;
+    buffer[1] |= EncodeSymbol(symbols_[7]) << 6;
+    buffer[1] |= EncodeSymbol(symbols_[8]) << 5;
+    buffer[1] |= EncodeSymbol(symbols_[9]) << 4;
+    buffer[1] |= EncodeSymbol(symbols_[10]) << 3;
+    buffer[1] |= EncodeSymbol(symbols_[11]) << 2;
+    buffer[1] |= EncodeSymbol(symbols_[12]) << 1;
+    buffer[1] |= EncodeSymbol(symbols_[13]);
+  }
+
+  static OneBitVectorChunk* ParseFrom(const uint8_t* data) {
+    OneBitVectorChunk* chunk = new OneBitVectorChunk();
+
+    size_t index = 0;
+    for (int i = 5; i >= 0; --i)  // Last 5 bits from first byte.
+      chunk->symbols_[index++] = DecodeSymbol((data[0] >> i) & 0x01);
+    for (int i = 7; i >= 0; --i)  // 8 bits from the last byte.
+      chunk->symbols_[index++] = DecodeSymbol((data[1] >> i) & 0x01);
+
+    return chunk;
+  }
+
+ private:
+  OneBitVectorChunk() {}
+
+  FeedbackPacket::StatusSymbol symbols_[kCapacity];
+};
+
+//  Two Bit Status Vector Chunk
+//
+//  0                   1
+//  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//  |T|S|       symbol list         |
+//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//
+//  T = 1
+//  S = 1
+//  symbol list = 7 entries of two bits each, see (Encode|Decode)Symbol
+
+class TwoBitVectorChunk : public PacketStatusChunk {
+ public:
+  static const int kCapacity = 7;
+
+  explicit TwoBitVectorChunk(
+      std::deque<FeedbackPacket::StatusSymbol>* symbols) {
+    size_t input_size = symbols->size();
+    for (size_t i = 0; i < kCapacity; ++i) {
+      if (i < input_size) {
+        symbols_[i] = symbols->front();
+        symbols->pop_front();
+      } else {
+        symbols_[i] = FeedbackPacket::StatusSymbol::kNotReceived;
+      }
+    }
+  }
+
+  virtual ~TwoBitVectorChunk() {}
+
+  uint16_t NumSymbols() const override { return kCapacity; }
+
+  void AppendSymbolsTo(
+      std::vector<FeedbackPacket::StatusSymbol>* vec) const override {
+    vec->insert(vec->end(), &symbols_[0], &symbols_[kCapacity]);
+  }
+
+  void WriteTo(uint8_t* buffer) const override {
+    buffer[0] = 0xC0;
+    buffer[0] |= EncodeSymbol(symbols_[0]) << 4;
+    buffer[0] |= EncodeSymbol(symbols_[1]) << 2;
+    buffer[0] |= EncodeSymbol(symbols_[2]);
+    buffer[1] = EncodeSymbol(symbols_[3]) << 6;
+    buffer[1] |= EncodeSymbol(symbols_[4]) << 4;
+    buffer[1] |= EncodeSymbol(symbols_[5]) << 2;
+    buffer[1] |= EncodeSymbol(symbols_[6]);
+  }
+
+  static TwoBitVectorChunk* ParseFrom(const uint8_t* buffer) {
+    TwoBitVectorChunk* chunk = new TwoBitVectorChunk();
+
+    chunk->symbols_[0] = DecodeSymbol((buffer[0] >> 4) & 0x03);
+    chunk->symbols_[1] = DecodeSymbol((buffer[0] >> 2) & 0x03);
+    chunk->symbols_[2] = DecodeSymbol(buffer[0] & 0x03);
+    chunk->symbols_[3] = DecodeSymbol((buffer[1] >> 6) & 0x03);
+    chunk->symbols_[4] = DecodeSymbol((buffer[1] >> 4) & 0x03);
+    chunk->symbols_[5] = DecodeSymbol((buffer[1] >> 2) & 0x03);
+    chunk->symbols_[6] = DecodeSymbol(buffer[1] & 0x03);
+
+    return chunk;
+  }
+
+ private:
+  TwoBitVectorChunk() {}
+
+  FeedbackPacket::StatusSymbol symbols_[kCapacity];
+};
+
+//  Two Bit Status Vector Chunk
+//
+//  0                   1
+//  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//  |T| S |       Run Length        |
+//  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//
+//  T = 0
+//  S = symbol, see (Encode|Decode)Symbol
+//  Run Length = Unsigned integer denoting the run length of the symbol
+
+class RunLengthChunk : public PacketStatusChunk {
+ public:
+  RunLengthChunk(FeedbackPacket::StatusSymbol symbol, size_t size)
+      : symbol_(symbol), size_(size) {
+    DCHECK_LE(size, 0x1FFFu);
+  }
+
+  virtual ~RunLengthChunk() {}
+
+  uint16_t NumSymbols() const override { return size_; }
+
+  void AppendSymbolsTo(
+      std::vector<FeedbackPacket::StatusSymbol>* vec) const override {
+    vec->insert(vec->end(), size_, symbol_);
+  }
+
+  void WriteTo(uint8_t* buffer) const override {
+    buffer[0] = EncodeSymbol(symbol_) << 5;  // Write S (T = 0 implicitly)
+    buffer[0] |= (size_ >> 8) & 0x1F;  // 5 most significant bits of run length.
+    buffer[1] = size_ & 0xFF;  // 8 least significant bits of run length.
+  }
+
+  static RunLengthChunk* ParseFrom(const uint8_t* buffer) {
+    DCHECK_EQ(0, buffer[0] & 0x80);
+    FeedbackPacket::StatusSymbol symbol = DecodeSymbol((buffer[0] >> 5) & 0x03);
+    uint16_t count = (static_cast<uint16_t>(buffer[0] & 0x1F) << 8) | buffer[1];
+
+    return new RunLengthChunk(symbol, count);
+  }
+
+ private:
+  const FeedbackPacket::StatusSymbol symbol_;
+  const size_t size_;
+};
+
+// Unwrap to a larger type, for easier handling of wraps.
+int64_t FeedbackPacket::Unwrap(uint16_t sequence_number) {
+  if (last_seq_ == -1)
+    return sequence_number;
+
+  int64_t delta = sequence_number - last_seq_;
+  if (IsNewerSequenceNumber(sequence_number,
+                            static_cast<uint16_t>(last_seq_))) {
+    if (delta < 0)
+      delta += (1 << 16);
+  } else if (delta > 0) {
+    delta -= (1 << 16);
+  }
+
+  return last_seq_ + delta;
+}
+
+void FeedbackPacket::WithPacketSenderSsrc(uint32_t ssrc) {
+  packet_sender_ssrc_ = ssrc;
+}
+
+void FeedbackPacket::WithMediaSourceSsrc(uint32_t ssrc) {
+  media_source_ssrc_ = ssrc;
+}
+
+void FeedbackPacket::WithBase(uint16_t base_sequence,
+                              int64_t ref_timestamp_us) {
+  DCHECK_EQ(-1, base_seq_);
+  DCHECK_NE(-1, ref_timestamp_us);
+  base_seq_ = base_sequence;
+  last_seq_ = base_sequence;
+  base_time_ = ref_timestamp_us / kBaseScaleFactor;
+  last_timestamp_ = base_time_ * kBaseScaleFactor;
+}
+
+void FeedbackPacket::WithFeedbackSequenceNumber(uint8_t feedback_sequence) {
+  feedback_seq_ = feedback_sequence;
+}
+
+bool FeedbackPacket::WithReceivedPacket(uint16_t sequence_number,
+                                        int64_t timestamp) {
+  DCHECK_NE(-1, base_seq_);
+  int64_t seq = Unwrap(sequence_number);
+  if (seq != base_seq_) {
+    if (seq <= last_seq_)

[stefan-webrtc, 2015/07/27 11:49:28]

if (seq != base_seq_ && seq <= last_seq_)
  return false;

[sprang_webrtc, 2015/07/28 08:28:54]

Done.

+      return false;
+  }
+
+  // Convert to ticks and round.
+  int64_t delta_full = timestamp - last_timestamp_;
+  delta_full +=
+      delta_full < 0 ? -(kDeltaScaleFactor / 2) : kDeltaScaleFactor / 2;
+  delta_full /= kDeltaScaleFactor;
+
+  int16_t delta = static_cast<int16_t>(delta_full);
+  // If larger than 16bit signed, we can't represent it - need new fb packet.
+  if (delta != delta_full) {
+    LOG(LS_WARNING) << "Delta value too large ( >= 2^16 ticks )";
+    return false;
+  }
+
+  StatusSymbol symbol;
+  if (delta >= 0 && delta <= 0xFF) {
+    symbol = StatusSymbol::kReceivedSmallDelta;
+  } else {
+    symbol = StatusSymbol::kReceivedLargeDelta;
+  }
+
+  if (!AddSymbol(symbol, seq))
+    return false;
+
+  receive_deltas_.push_back(delta);
+  last_timestamp_ += delta * kDeltaScaleFactor;
+  return true;
+}
+
+// Add a symbol for a received packet, with the given sequence number. This
+// method will add any "packet not received" symbols needed before this one.
+bool FeedbackPacket::AddSymbol(StatusSymbol symbol, int64_t seq) {
+  while (last_seq_ < seq - 1) {
+    if (!Encode(StatusSymbol::kNotReceived))
+      return false;
+    ++last_seq_;
+  }
+
+  if (!Encode(symbol))
+    return false;
+
+  last_seq_ = seq;
+  return true;
+}
+
+// Append a symbol to the internal symbol vector. If the new state cannot be
+// represented using a single status chunk, a chunk will first be emitted and
+// the associated symbols removed from the internal symbol vector.
+bool FeedbackPacket::Encode(StatusSymbol symbol) {
+  if (last_seq_ - base_seq_ + 1 > 0xFFFF) {
+    LOG(LS_WARNING) << "Packet status count too large ( >= 2^16 )";
+    return false;
+  }
+
+  bool is_two_bit;
+  int delta_size;
+  switch (symbol) {
+    case StatusSymbol::kReceivedSmallDelta:
+      delta_size = 1;
+      is_two_bit = false;
+      break;
+    case StatusSymbol::kReceivedLargeDelta:
+      delta_size = 2;
+      is_two_bit = true;
+      break;
+    case StatusSymbol::kNotReceived:
+      is_two_bit = false;
+      delta_size = 0;
+      break;
+    default:
+      RTC_NOTREACHED();
+      return false;
+  }
+
+  if (symbol_vec_.empty()) {
+    if (size_bytes_ + delta_size + kChunkSizeBytes > kMaxSizeBytes)
+      return false;
+
+    symbol_vec_.push_back(symbol);
+    vec_needs_two_bit_symbols_ = is_two_bit;
+    first_symbol_cardinality_ = 1;
+    size_bytes_ += delta_size + kChunkSizeBytes;
+    return true;
+  }
+  if (size_bytes_ + delta_size > kMaxSizeBytes)
+    return false;
+
+  // Capacity, in number of symbols, that a vector chunk could hold.
+  size_t capacity = vec_needs_two_bit_symbols_ ? kTwoBitVectorCapacity
+                                               : kOneBitVectorCapacity;
+
+  // first_symbol_cardinality_ is the number of times the first symbol in
+  // symbol_vec is repeated. So if that is equal to the size of symbol_vec,
+  // there is only one kind of symbol - we can potentially RLE encode it.
+  // If we have less than (capacity) symbols in symbol_vec, we can't know
+  // for certain this will be RLE-encoded; if a different symbol is added
+  // these symbols will be needed to emit a vector chunk instead. However,
+  // if first_symbol_cardinality_ > capacity, then we cannot encode the
+  // current state as a vector chunk - we must first emit symbol_vec as an
+  // RLE-chunk and then add the new symbol.
+  bool rle_candidate = symbol_vec_.size() == first_symbol_cardinality_ ||
+                       first_symbol_cardinality_ > capacity;
+  if (rle_candidate) {
+    if (symbol_vec_.back() == symbol) {
+      ++first_symbol_cardinality_;
+      if (first_symbol_cardinality_ <= capacity) {
+        symbol_vec_.push_back(symbol);
+      } else if (first_symbol_cardinality_ == kRunLengthCapacity) {
+        // Max length for an RLE-chunk reached.
+        EmitRunLengthChunk();
+      }
+      size_bytes_ += delta_size;
+      return true;
+    } else {
+      // New symbol does not match what's already in symbol_vec.
+      if (first_symbol_cardinality_ > capacity) {
+        // Symbols in symbol_vec can only be RLE-encoded. Emit the RLE-chunk
+        // and re-add input. symbol_vec is then guaranteed to have room for the
+        // symbol, so recursion cannot continue.
+        EmitRunLengthChunk();
+        return Encode(symbol);
+      }
+      // Fall through and treat state as non RLE-candidate.
+    }
+  }
+
+  // If this code point is reached, symbols in symbol_vec cannot be RLE-encoded.
+
+  if (is_two_bit && !vec_needs_two_bit_symbols_) {
+    // If the symbols in symbol_vec can be encoded using a one-bit chunk but
+    // the input symbol cannot, first check if we can simply change target type.
+    vec_needs_two_bit_symbols_ = true;
+    if (symbol_vec_.size() >= kTwoBitVectorCapacity) {
+      // symbol_vec contains more symbols than we can encode in a single
+      // two-bit chunk. Emit a new vector append to the remains, if any.
+      if (size_bytes_ + delta_size + kChunkSizeBytes > kMaxSizeBytes)
+        return false;
+      EmitVectorChunk();
+      // If symbol_vec isn't empty after emitting a vector chunk, we need to
+      // account for chunk size (otherwise handled by Encode method).
+      if (!symbol_vec_.empty())
+        size_bytes_ += kChunkSizeBytes;
+      return Encode(symbol);
+    }
+    // symbol_vec symbols fit within a single two-bit vector chunk.
+    capacity = kTwoBitVectorCapacity;
+  }
+
+  symbol_vec_.push_back(symbol);
+  if (symbol_vec_.size() == capacity)
+    EmitVectorChunk();
+
+  size_bytes_ += delta_size;
+  return true;
+}
+
+// Upon packet completion, emit any remaining symbols in symbol_vec that have
+// not yet been emitted in a status chunk.
+void FeedbackPacket::EmitRemaining() {
+  if (symbol_vec_.empty())
+    return;
+
+  size_t capacity = vec_needs_two_bit_symbols_ ? kTwoBitVectorCapacity
+                                               : kOneBitVectorCapacity;
+  if (first_symbol_cardinality_ > capacity) {
+    EmitRunLengthChunk();
+  } else {
+    EmitVectorChunk();
+  }
+}
+
+void FeedbackPacket::EmitVectorChunk() {
+  if (vec_needs_two_bit_symbols_) {
+    status_chunks_.push_back(new TwoBitVectorChunk(&symbol_vec_));
+  } else {
+    status_chunks_.push_back(new OneBitVectorChunk(&symbol_vec_));
+  }
+  first_symbol_cardinality_ = 1;
+  for (size_t i = 1; i < symbol_vec_.size(); ++i) {

[stefan-webrtc, 2015/07/27 11:49:28]

Comment: Update first symbol cardinality for the remaining symbols.

[sprang_webrtc, 2015/07/28 08:28:54]

Done.

+    if (symbol_vec_[i] != symbol_vec_[0])
+      break;
+    ++first_symbol_cardinality_;
+  }
+}
+
+void FeedbackPacket::EmitRunLengthChunk() {
+  DCHECK_GE(first_symbol_cardinality_, symbol_vec_.size());
+  status_chunks_.push_back(
+      new RunLengthChunk(symbol_vec_.front(), first_symbol_cardinality_));
+  symbol_vec_.clear();
+}
+
+size_t FeedbackPacket::BlockLength() const {
+  return size_bytes_;
+}
+
+uint16_t FeedbackPacket::GetBaseSequence() const {
+  return base_seq_;
+}
+
+int32_t FeedbackPacket::GetBaseTime() const {
+  return static_cast<int32_t>(base_time_ & 0x00FFFFFF);
+}
+
+int64_t FeedbackPacket::GetBaseTimeUs() const {
+  return GetBaseTime() * kBaseScaleFactor;
+}
+
+std::vector<FeedbackPacket::StatusSymbol> FeedbackPacket::GetStatusVector()
+    const {
+  std::vector<FeedbackPacket::StatusSymbol> symbols;
+  for (PacketStatusChunk* chunk : status_chunks_)
+    chunk->AppendSymbolsTo(&symbols);
+  int64_t status_count = last_seq_ - base_seq_ + 1;
+  // If packet ends with a vector chunk, it may contain extraneous "packet not
+  // received"-symbols at the end. Crop any such symbols.
+  symbols.erase(symbols.begin() + status_count, symbols.end());
+  return symbols;
+}
+
+std::vector<int16_t> FeedbackPacket::GetReceiveDeltas() const {
+  return receive_deltas_;
+}
+
+std::vector<int64_t> FeedbackPacket::GetReceiveDeltasUs() const {
+  if (receive_deltas_.empty())
+    return std::vector<int64_t>();
+
+  std::vector<int64_t> us_deltas;
+  for (int16_t delta : receive_deltas_)
+    us_deltas.push_back(static_cast<int64_t>(delta) * kDeltaScaleFactor);
+
+  return us_deltas;
+}
+
+// Serialize packet.
+bool FeedbackPacket::Create(uint8_t* packet,
+                            size_t* position,
+                            size_t max_length,
+                            PacketReadyCallback* callback) const {
+  if (base_seq_ == -1)
+    return false;
+
+  while (*position + size_bytes_ > max_length) {
+    if (!OnBufferFull(packet, position, callback))
+      return false;
+  }
+
+  CreateHeader(kFeedbackMessageType, kPayloadType, HeaderLength(), packet,
+               position);
+  ByteWriter<uint32_t>::WriteBigEndian(&packet[*position], packet_sender_ssrc_);
+  *position += 4;
+  ByteWriter<uint32_t>::WriteBigEndian(&packet[*position], media_source_ssrc_);
+  *position += 4;
+
+  DCHECK_LE(base_seq_, 0xFFFF);
+  ByteWriter<uint16_t>::WriteBigEndian(&packet[*position], base_seq_);
+  *position += 2;
+
+  int64_t status_count = last_seq_ - base_seq_ + 1;
+  DCHECK_LE(status_count, 0xFFFF);
+  ByteWriter<uint16_t>::WriteBigEndian(&packet[*position], status_count);
+  *position += 2;
+
+  ByteWriter<int32_t, 3>::WriteBigEndian(&packet[*position],
+                                         static_cast<int16_t>(base_time_));
+  *position += 3;
+
+  packet[(*position)++] = feedback_seq_;
+
+  // TODO(sprang): Get rid of this cast.
+  const_cast<FeedbackPacket*>(this)->EmitRemaining();
+  for (PacketStatusChunk* chunk : status_chunks_) {
+    chunk->WriteTo(&packet[*position]);
+    *position += 2;
+  }
+
+  for (int16_t delta : receive_deltas_) {
+    if (delta >= 0 && delta <= 0xFF) {
+      packet[(*position)++] = delta;
+    } else {
+      ByteWriter<int16_t>::WriteBigEndian(&packet[*position], delta);
+      *position += 2;
+    }
+  }
+
+  while ((*position % 4) != 0)
+    packet[(*position)++] = 0;
+
+  return true;
+}
+
+//    Message format
+//
+//     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
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |V=2|P|  FMT=15 |    PT=205     |           length              |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |                     SSRC of packet sender                     |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |                      SSRC of media source                     |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |      base sequence number     |      packet status count      |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |                 reference time                | fb pkt. count |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |          packet chunk         |         packet chunk          |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    .                                                               .
+//    .                                                               .
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |         packet chunk          |  recv delta   |  recv delta   |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    .                                                               .
+//    .                                                               .
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//    |           recv delta          |  recv delta   | zero padding  |
+//    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+// De-serialize packet.
+rtc::scoped_ptr<FeedbackPacket> FeedbackPacket::ParseFrom(const uint8_t* buffer,
+                                                          size_t length) {
+  rtc::scoped_ptr<FeedbackPacket> packet(new FeedbackPacket());
+
+  if (length < kMinSizeBytes) {
+    LOG(LS_WARNING) << "Buffer too small (" << length
+                    << " bytes) to fit a "
+                       "FeedbackPacket. Minimum size = " << kMinSizeBytes;
+    return nullptr;
+  }
+
+  size_t packet_size_words =
+      ByteReader<uint16_t>::ReadBigEndian(&buffer[2]) + 1;
+  if (length < packet_size_words * 4) {
+    LOG(LS_WARNING) << "Buffer too small (" << length
+                    << " bytes) to fit a FeedbackPacket of "
+                    << packet_size_words << " 32bit words.";
+    return nullptr;
+  }
+
+  // TODO(sprang): Break this out and generalize when implementing parsing of
+  // other RtcpPacket subclasses.
+
+  const uint8_t kRtcpVersion = 2;
+  uint8_t version = buffer[0] >> 6;
+  if (version != kRtcpVersion) {
+    LOG(LS_WARNING) << "Invalid RTCP header: Version must be " << kRtcpVersion
+                    << " but was " << version;
+    return nullptr;
+  }
+
+  bool has_padding = (buffer[0] & 0x20) != 0;
+
+  uint8_t format = buffer[0] & 0x1F;
+  if (format != kFeedbackMessageType) {
+    LOG(LS_WARNING) << "Invalid RTCP header: FMT must be "
+                    << kFeedbackMessageType << " but was " << format;
+    return nullptr;
+  }
+
+  uint8_t payload_type = buffer[1];
+  if (payload_type != kPayloadType) {
+    LOG(LS_WARNING) << "Invalid RTCP header: PT must be " << kPayloadType
+                    << " but was " << payload_type;
+    return nullptr;
+  }
+
+  size_t payload_size = packet_size_words * 4;
+  if (has_padding) {
+    uint8_t padding_bytes = buffer[payload_size - 1];
+    if (payload_size < kMinSizeBytes + padding_bytes) {
+      LOG(LS_WARNING) << "Invalid RTCP header: Too many padding bytes ("
+                      << padding_bytes << ") for a packet size of "
+                      << payload_size << "bytes.";
+      return nullptr;
+    }
+    payload_size -= padding_bytes;
+  }
+
+  packet->packet_sender_ssrc_ = ByteReader<uint32_t>::ReadBigEndian(&buffer[4]);
+  packet->media_source_ssrc_ = ByteReader<uint32_t>::ReadBigEndian(&buffer[8]);
+  packet->base_seq_ = ByteReader<uint16_t>::ReadBigEndian(&buffer[12]);
+  uint16_t num_packets = ByteReader<uint16_t>::ReadBigEndian(&buffer[14]);
+  packet->base_time_ = ByteReader<int32_t, 3>::ReadBigEndian(&buffer[16]);
+  packet->feedback_seq_ = buffer[19];
+  size_t index = 20;
+
+  if (num_packets == 0) {
+    LOG(LS_WARNING) << "Empty feedback messages not allowed.";
+    return nullptr;
+  }
+  packet->last_seq_ = packet->base_seq_ + num_packets - 1;
+
+  size_t packets_read = 0;
+  while (packets_read < num_packets) {
+    if (index + 2 > payload_size) {
+      LOG(LS_WARNING) << "Buffer overflow while parsing packet.";
+      return nullptr;
+    }
+
+    PacketStatusChunk* chunk =
+        ParseChunk(&buffer[index], num_packets - packets_read);
+    if (chunk == nullptr)
+      return nullptr;
+
+    index += 2;
+    packet->status_chunks_.push_back(chunk);
+    packets_read += chunk->NumSymbols();
+  }
+
+  std::vector<StatusSymbol> symbols = packet->GetStatusVector();
+
+  DCHECK_EQ(num_packets, symbols.size());
+
+  for (StatusSymbol symbol : symbols) {
+    switch (symbol) {
+      case StatusSymbol::kReceivedSmallDelta:
+        if (index + 1 > payload_size) {
+          LOG(LS_WARNING) << "Buffer overflow while parsing packet.";
+          return nullptr;
+        }
+        packet->receive_deltas_.push_back(buffer[index]);
+        ++index;
+        break;
+      case StatusSymbol::kReceivedLargeDelta:
+        if (index + 2 > payload_size) {
+          LOG(LS_WARNING) << "Buffer overflow while parsing packet.";
+          return nullptr;
+        }
+        packet->receive_deltas_.push_back(
+            ByteReader<int16_t>::ReadBigEndian(&buffer[index]));
+        index += 2;
+        break;
+      default:
+        continue;
+    }
+  }
+
+  DCHECK_GE(index, payload_size - 3);
+  DCHECK_LE(index, payload_size);
+
+  return packet;
+}
+
+PacketStatusChunk* FeedbackPacket::ParseChunk(const uint8_t* buffer,
+                                              size_t max_size) {
+  if (buffer[0] & 0x80) {
+    // First bit set => vector chunk.
+    std::deque<StatusSymbol> symbols;
+    if (buffer[0] & 0x40) {
+      // Second bit set => two bits per symbol vector.
+      return TwoBitVectorChunk::ParseFrom(buffer);
+    }
+
+    // Second bit not set => one bit per symbol vector.
+    return OneBitVectorChunk::ParseFrom(buffer);
+  }
+
+  // First bit not set => RLE chunk.
+  RunLengthChunk* rle_chunk = RunLengthChunk::ParseFrom(buffer);
+  if (rle_chunk->NumSymbols() > max_size) {
+    LOG(LS_WARNING) << "Header/body mismatch. "
+                       "RLE block of size " << rle_chunk->NumSymbols()
+                    << " but only " << max_size << " left to read.";
+    delete rle_chunk;
+    return nullptr;
+  }
+  return rle_chunk;
+}
+
+}  // namespace rtcp
+}  // namespace webrtc