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 {
+
+class FeedbackPacket::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;
+    case FeedbackPacket::StatusSymbol::kReceivedUntimed:
+      return 3;
+    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;
+    case 3:
+      return FeedbackPacket::StatusSymbol::kReceivedUntimed;
+    default:
+      RTC_NOTREACHED();
+      return FeedbackPacket::StatusSymbol::kNotReceived;
+  }
+}
+
+FeedbackPacket::FeedbackPacket()
+    : base_seq_(-1),
+      base_delta_(-1),
+      feedback_seq_(0),
+      last_seq_(-1),
+      last_timestamp_(-1),
+      first_symbol_cardinality_(0),
+      vec_needs_two_bit_symbols_(false),
+      size_bytes_(8) {
+}
+
+FeedbackPacket::~FeedbackPacket() {
+  for (PacketStatusChunk* chunk : status_chunks_)
+    delete chunk;
+}
+
+class OneBitVectorChunk : public FeedbackPacket::PacketStatusChunk {
+ public:
+  explicit OneBitVectorChunk(
+      std::deque<FeedbackPacket::StatusSymbol>* symbols) {
+    size_t input_size = symbols->size();
+    for (size_t i = 0; i < 14; ++i) {

[stefan-webrtc, 2015/06/16 08:29:13]

What is 14 here? I think it's worth naming it and explaining why it is 14.

[sprang_webrtc, 2015/06/22 12:20:10]

Done.

+      if (i < input_size) {
+        symbols_[i] = symbols->front();
+        symbols->pop_front();
+      } else {
+        symbols_[i] = FeedbackPacket::StatusSymbol::kNotReceived;
+      }
+    }
+  }
+
+  virtual ~OneBitVectorChunk() {}
+
+  uint16_t NumSymbols() const override { return 14; }
+
+  void AppendSymbolsTo(
+      std::vector<FeedbackPacket::StatusSymbol>* vec) const override {
+    vec->insert(vec->end(), &symbols_[0], &symbols_[14]);
+  }
+
+  void WriteTo(uint8_t* buffer) const override {
+    buffer[0] = 0x80;
+    buffer[0] |= EncodeSymbol(symbols_[0]) << 5;
+    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)
+      chunk->symbols_[index++] = DecodeSymbol((data[0] >> i) & 0x01);
+    for (int i = 7; i >= 0; --i)
+      chunk->symbols_[index++] = DecodeSymbol((data[1] >> i) & 0x01);

[stefan-webrtc, 2015/06/16 08:29:13]

Probably worth mentioning why this is done in two steps (5->0 and 7->0).
Basically, what are you reading in the first step and what in the second?

[sprang_webrtc, 2015/06/22 12:20:10]

Done.

+
+    return chunk;
+  }
+
+ private:
+  OneBitVectorChunk() {}
+
+  FeedbackPacket::StatusSymbol symbols_[14];
+};
+
+class TwoBitVectorChunk : public FeedbackPacket::PacketStatusChunk {
+ public:
+  explicit TwoBitVectorChunk(
+      std::deque<FeedbackPacket::StatusSymbol>* symbols) {
+    size_t input_size = symbols->size();
+    for (size_t i = 0; i < 7; ++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 7; }
+
+  void AppendSymbolsTo(
+      std::vector<FeedbackPacket::StatusSymbol>* vec) const override {
+    vec->insert(vec->end(), &symbols_[0], &symbols_[7]);
+  }
+
+  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_[7];

[stefan-webrtc, 2015/06/16 08:29:13]

Why 7? Name the constant and explain.

[sprang_webrtc, 2015/06/22 12:20:10]

Done.

+};
+
+class RunLengthChunk : public FeedbackPacket::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 {

[stefan-webrtc, 2015/06/16 08:29:13]

I find this method complicated enough to deserve a comment. :)

[sprang_webrtc, 2015/06/22 12:20:10]

Done.

+    buffer[0] = EncodeSymbol(symbol_) << 5;
+    buffer[0] |= (size_ >> 8) & 0x1F;
+    buffer[1] = size_ & 0xFF;
+  }
+
+  static RunLengthChunk* ParseFrom(const uint8_t* buffer) {

[stefan-webrtc, 2015/06/16 08:29:13]

Same with this.

[sprang_webrtc, 2015/06/22 12:20:10]

Should be ok with changes above?

[stefan-webrtc, 2015/07/14 12:14:54]

Yes

[sprang_webrtc, 2015/07/16 11:12:10]

Acknowledged.

+    FeedbackPacket::StatusSymbol symbol = DecodeSymbol((buffer[0] >> 5) & 0x03);
+    uint16_t count = ((buffer[0] & 0x1F) << 8) | buffer[1];
+
+    return new RunLengthChunk(symbol, count);
+  }
+
+ private:
+  const FeedbackPacket::StatusSymbol symbol_;
+  const size_t size_;
+};
+
+int64_t FeedbackPacket::Unwrap(uint16_t sequence_number) {
+  if (last_seq_ == -1)
+    return sequence_number;
+
+  int64_t delta = sequence_number - (last_seq_ & 0xFFFF);
+  if (delta <= -0x8FFF) {

[stefan-webrtc, 2015/06/16 08:29:13]

Shouldn't this be -0x7FFF?

I wonder if we could rewrite this using IsNewerSequenceNumber() or
LatestSequenceNumber() in module_common_types.h? I think it's worth a lot to
reuse this type of code.

[sprang_webrtc, 2015/06/22 12:20:10]

Done.

+    // Wrap forward.
+    delta += (1 << 16);
+  } else if (delta > 0x8FFF) {
+    // Wrap backwards.
+    delta -= (1 << 16);
+  }
+
+  return last_seq_ + delta;
+}
+
+void FeedbackPacket::WithBase(uint16_t base_sequence,
+                              int64_t prev_timestamp_us,
+                              int64_t ref_timestamp_us) {

[stefan-webrtc, 2015/06/16 08:29:13]

Would base_timestamp_us be better? Or is that not what it means?

[sprang_webrtc, 2015/06/22 12:20:10]

Changed to use absolute ref time, as per discussion.

+  DCHECK_EQ(-1, base_seq_);
+  DCHECK_NE(-1, ref_timestamp_us);
+  base_seq_ = base_sequence;
+  last_seq_ = base_sequence;
+  if (prev_timestamp_us != -1) {
+    base_delta_ = (ref_timestamp_us - prev_timestamp_us) / kBaseScaleFactor;
+    DCHECK_EQ(base_delta_, static_cast<int16_t>(base_delta_));
+
+    // Include precision loss in order to avoid drift.

[stefan-webrtc, 2015/07/14 12:14:54]

Keep this comment, I found it really helpful.

+    last_timestamp_ = prev_timestamp_us + (base_delta_ * kBaseScaleFactor);
+  } else {
+    base_delta_ = 0;
+    last_timestamp_ = ref_timestamp_us;
+  }
+}
+
+void FeedbackPacket::WithFeedbackSequenceNumber(uint8_t feedback_sequence) {
+  feedback_seq_ = feedback_sequence;
+}
+
+bool FeedbackPacket::WithUntimedPacket(uint16_t sequence_number) {
+  DCHECK_NE(-1, base_seq_);
+  int64_t seq = Unwrap(sequence_number);
+  DCHECK_GE(seq, last_seq_);
+  return AddSymbol(StatusSymbol::kReceivedUntimed, sequence_number);
+}
+
+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_)
+      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;
+}
+
+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;
+}
+
+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 = (symbol == StatusSymbol::kReceivedLargeDelta ||
+                     symbol == StatusSymbol::kReceivedUntimed);
+  int delta_size;
+  switch (symbol) {
+    case StatusSymbol::kReceivedSmallDelta:
+      delta_size = 1;
+      break;
+    case StatusSymbol::kReceivedLargeDelta:
+      delta_size = 2;
+      break;
+    default:
+      delta_size = 0;
+  }
+
+  if (symbol_vec_.empty()) {
+    if (size_bytes_ + delta_size + 2 > kMaxSizeBytes)
+      return false;
+
+    symbol_vec_.push_back(symbol);
+    vec_needs_two_bit_symbols_ = is_two_bit;
+    first_symbol_cardinality_ = 1;
+    size_bytes_ += delta_size + 2;
+    return true;
+  }
+  if (size_bytes_ + delta_size > kMaxSizeBytes)
+    return false;
+
+  size_t capacity = vec_needs_two_bit_symbols_ ? kTwoBitVectorCapacity
+                                               : kOneBitVectorCapacity;
+  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) {
+        EmitRunLengthChunk();
+      }
+      size_bytes_ += delta_size;
+      return true;
+    } else if (first_symbol_cardinality_ > capacity) {
+      // Convert from run length to vector.
+      EmitRunLengthChunk();
+      return Encode(symbol);
+    }
+  }
+
+  if (is_two_bit && !vec_needs_two_bit_symbols_) {
+    if (symbol_vec_.size() >= kTwoBitVectorCapacity) {
+      if (size_bytes_ + delta_size + 2 > kMaxSizeBytes)
+        return false;
+      EmitVectorChunk();
+      return Encode(symbol);
+    }
+    vec_needs_two_bit_symbols_ = true;
+    capacity = kTwoBitVectorCapacity;
+  }
+
+  symbol_vec_.push_back(symbol);
+  size_t symbol_vec_size = symbol_vec_.size();
+  if (symbol_vec_size == capacity)
+    EmitVectorChunk();
+
+  size_bytes_ += delta_size;
+  return true;
+}
+
+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_));
+  }
+}
+
+uint16_t FeedbackPacket::GetBaseSequence() const {
+  return base_seq_;
+}
+
+int16_t FeedbackPacket::GetBaseDelta() const {
+  return base_delta_;
+}
+
+int64_t FeedbackPacket::GetBaseDeltaUs() const {
+  return base_delta_ * 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;
+  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 {
+  std::vector<int64_t> us_deltas;
+  if (receive_deltas_.empty()) {
+    us_deltas.push_back(base_delta_ * kBaseScaleFactor);
+    return us_deltas;
+  }
+
+  for (int16_t delta : receive_deltas_)
+    us_deltas.push_back(static_cast<int64_t>(delta) * kDeltaScaleFactor);
+  us_deltas[0] += base_delta_ * kBaseScaleFactor;
+
+  return us_deltas;
+}
+
+void FeedbackPacket::EmitRunLengthChunk() {
+  status_chunks_.push_back(
+      new RunLengthChunk(symbol_vec_.front(), first_symbol_cardinality_));
+  symbol_vec_.clear();
+}
+
+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;
+  }
+
+  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;
+
+  DCHECK_LE(base_delta_, 0xFFFF);
+  ByteWriter<int16_t>::WriteBigEndian(&packet[*position], base_delta_);
+  *position += 2;
+
+  packet[(*position)++] = feedback_seq_;
+
+  uint16_t size_words = (size_bytes_ + 3) / 4;
+  size_words -= 2;  // Excluding header.
+  DCHECK_LE(size_words, 0xFF);
+  packet[(*position)++] = size_words;
+
+  // 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;
+}
+
+rtc::scoped_ptr<FeedbackPacket> FeedbackPacket::ParseFrom(const uint8_t* buffer,
+                                                          size_t length) {
+  rtc::scoped_ptr<FeedbackPacket> packet(new FeedbackPacket());
+
+  size_t packet_size_words = 0;
+  if (length < kMinSizeBytes ||
+      length < (packet_size_words = buffer[7] * 4) + 8) {
+    LOG(LS_WARNING) << "Buffer too small (" << length
+                    << " bytes) to parse feedback packet of "
+                    << (packet_size_words * 4 + 8) << "bytes.";
+    return nullptr;
+  }
+
+  packet->base_seq_ = ByteReader<uint16_t>::ReadBigEndian(&buffer[0]);
+  uint16_t num_packets = ByteReader<uint16_t>::ReadBigEndian(&buffer[2]);
+  packet->base_delta_ = ByteReader<int16_t>::ReadBigEndian(&buffer[4]);
+  packet->feedback_seq_ = buffer[6];
+  size_t index = 8;
+
+  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 > length) {
+      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 > length) {
+          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 > length) {
+          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, length - 3);
+  DCHECK_LE(index, length);
+
+  return packet;
+}
+
+FeedbackPacket::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