text: // Copyright (c) 2012 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  #include "net/quic/quic_framer.h"
  
  #include <stdint.h>
  
  #include "base/compiler_specific.h"
  #include "base/logging.h"
  #include "base/stl_util.h"
  #include "net/quic/crypto/crypto_framer.h"
  #include "net/quic/crypto/crypto_handshake_message.h"
  #include "net/quic/crypto/crypto_protocol.h"
  #include "net/quic/crypto/quic_decrypter.h"
  #include "net/quic/crypto/quic_encrypter.h"
  #include "net/quic/quic_data_reader.h"
  #include "net/quic/quic_data_writer.h"
  #include "net/quic/quic_flags.h"
  #include "net/quic/quic_socket_address_coder.h"
  #include "net/quic/quic_utils.h"
  
  using base::StringPiece;
  using std::map;
  using std::max;
  using std::min;
  using std::numeric_limits;
  using std::string;
  
  namespace net {
  
  namespace {
  
  // Mask to select the lowest 48 bits of a packet number.
  const QuicPacketNumber k6ByteSequenceNumberMask = UINT64_C(0x0000FFFFFFFFFFFF);
  const QuicPacketNumber k4ByteSequenceNumberMask = UINT64_C(0x00000000FFFFFFFF);
  const QuicPacketNumber k2ByteSequenceNumberMask = UINT64_C(0x000000000000FFFF);
  const QuicPacketNumber k1ByteSequenceNumberMask = UINT64_C(0x00000000000000FF);
  
  const QuicConnectionId k1ByteConnectionIdMask = UINT64_C(0x00000000000000FF);
  const QuicConnectionId k4ByteConnectionIdMask = UINT64_C(0x00000000FFFFFFFF);
  
  // Number of bits the packet number length bits are shifted from the right
  // edge of the public header.
  const uint8_t kPublicHeaderSequenceNumberShift = 4;
  
  // New Frame Types, QUIC v. >= 10:
  // There are two interpretations for the Frame Type byte in the QUIC protocol,
  // resulting in two Frame Types: Special Frame Types and Regular Frame Types.
  //
  // Regular Frame Types use the Frame Type byte simply. Currently defined
  // Regular Frame Types are:
  // Padding            : 0b 00000000 (0x00)
  // ResetStream        : 0b 00000001 (0x01)
  // ConnectionClose    : 0b 00000010 (0x02)
  // GoAway             : 0b 00000011 (0x03)
  // WindowUpdate       : 0b 00000100 (0x04)
  // Blocked            : 0b 00000101 (0x05)
  //
  // Special Frame Types encode both a Frame Type and corresponding flags
  // all in the Frame Type byte. Currently defined Special Frame Types are:
  // Stream             : 0b 1xxxxxxx
  // Ack                : 0b 01xxxxxx
  //
  // Semantics of the flag bits above (the x bits) depends on the frame type.
  
  // Masks to determine if the frame type is a special use
  // and for specific special frame types.
  const uint8_t kQuicFrameTypeSpecialMask = 0xE0;  // 0b 11100000
  const uint8_t kQuicFrameTypeStreamMask = 0x80;
  const uint8_t kQuicFrameTypeAckMask = 0x40;
  
  // Stream frame relative shifts and masks for interpreting the stream flags.
  // StreamID may be 1, 2, 3, or 4 bytes.
  const uint8_t kQuicStreamIdShift = 2;
  const uint8_t kQuicStreamIDLengthMask = 0x03;
  
  // Offset may be 0, 2, 3, 4, 5, 6, 7, 8 bytes.
  const uint8_t kQuicStreamOffsetShift = 3;
  const uint8_t kQuicStreamOffsetMask = 0x07;
  
  // Data length may be 0 or 2 bytes.
  const uint8_t kQuicStreamDataLengthShift = 1;
  const uint8_t kQuicStreamDataLengthMask = 0x01;
  
  // Fin bit may be set or not.
  const uint8_t kQuicStreamFinShift = 1;
  const uint8_t kQuicStreamFinMask = 0x01;
  
  // packet number size shift used in AckFrames.
  const uint8_t kQuicSequenceNumberLengthShift = 2;
  
  // Acks may be truncated.
  const uint8_t kQuicAckTruncatedShift = 1;
  const uint8_t kQuicAckTruncatedMask = 0x01;
  
  // Acks may not have any nacks.
  const uint8_t kQuicHasNacksMask = 0x01;
  
  // Returns the absolute value of the difference between |a| and |b|.
  QuicPacketNumber Delta(QuicPacketNumber a, QuicPacketNumber b) {
    // Since these are unsigned numbers, we can't just return abs(a - b)
    if (a < b) {
      return b - a;
    }
    return a - b;
  }
  
  QuicPacketNumber ClosestTo(QuicPacketNumber target,
                             QuicPacketNumber a,
                             QuicPacketNumber b) {
    return (Delta(target, a) < Delta(target, b)) ? a : b;
  }
  
  QuicPacketNumberLength ReadSequenceNumberLength(uint8_t flags) {
    switch (flags & PACKET_FLAGS_6BYTE_PACKET) {
      case PACKET_FLAGS_6BYTE_PACKET:
        return PACKET_6BYTE_PACKET_NUMBER;
      case PACKET_FLAGS_4BYTE_PACKET:
        return PACKET_4BYTE_PACKET_NUMBER;
      case PACKET_FLAGS_2BYTE_PACKET:
        return PACKET_2BYTE_PACKET_NUMBER;
      case PACKET_FLAGS_1BYTE_PACKET:
        return PACKET_1BYTE_PACKET_NUMBER;
      default:
        LOG(DFATAL) << "Unreachable case statement.";
        return PACKET_6BYTE_PACKET_NUMBER;
    }
  }
  
  }  // namespace
  
  QuicFramer::QuicFramer(const QuicVersionVector& supported_versions,
                         QuicTime creation_time,
                         Perspective perspective)
      : visitor_(nullptr),
        entropy_calculator_(nullptr),
        error_(QUIC_NO_ERROR),
        last_packet_number_(0),
        last_path_id_(kInvalidPathId),
        last_serialized_connection_id_(0),
        supported_versions_(supported_versions),
        decrypter_level_(ENCRYPTION_NONE),
        alternative_decrypter_level_(ENCRYPTION_NONE),
        alternative_decrypter_latch_(false),
        perspective_(perspective),
        validate_flags_(true),
        creation_time_(creation_time),
        last_timestamp_(QuicTime::Delta::Zero()) {
    DCHECK(!supported_versions.empty());
    quic_version_ = supported_versions_[0];
    decrypter_.reset(QuicDecrypter::Create(kNULL));
    encrypter_[ENCRYPTION_NONE].reset(QuicEncrypter::Create(kNULL));
  }
  
  QuicFramer::~QuicFramer() {}
  
  // static
  size_t QuicFramer::GetMinStreamFrameSize(QuicStreamId stream_id,
                                           QuicStreamOffset offset,
                                           bool last_frame_in_packet,
                                           InFecGroup is_in_fec_group) {
    bool no_stream_frame_length =
        last_frame_in_packet && is_in_fec_group == NOT_IN_FEC_GROUP;
    return kQuicFrameTypeSize + GetStreamIdSize(stream_id) +
           GetStreamOffsetSize(offset) +
           (no_stream_frame_length ? 0 : kQuicStreamPayloadLengthSize);
  }
  
  // static
  size_t QuicFramer::GetMinAckFrameSize(
      QuicPacketNumberLength largest_observed_length) {
    return kQuicFrameTypeSize + kQuicEntropyHashSize + largest_observed_length +
           kQuicDeltaTimeLargestObservedSize;
  }
  
  // static
  size_t QuicFramer::GetStopWaitingFrameSize(
      QuicPacketNumberLength packet_number_length) {
    return kQuicFrameTypeSize + kQuicEntropyHashSize + packet_number_length;
  }
  
  // static
  size_t QuicFramer::GetMinRstStreamFrameSize() {
    return kQuicFrameTypeSize + kQuicMaxStreamIdSize + kQuicMaxStreamOffsetSize +
           kQuicErrorCodeSize + kQuicErrorDetailsLengthSize;
  }
  
  // static
  size_t QuicFramer::GetRstStreamFrameSize() {
    return kQuicFrameTypeSize + kQuicMaxStreamIdSize + kQuicMaxStreamOffsetSize +
           kQuicErrorCodeSize;
  }
  
  // static
  size_t QuicFramer::GetMinConnectionCloseFrameSize() {
    return kQuicFrameTypeSize + kQuicErrorCodeSize + kQuicErrorDetailsLengthSize;
  }
  
  // static
  size_t QuicFramer::GetMinGoAwayFrameSize() {
    return kQuicFrameTypeSize + kQuicErrorCodeSize + kQuicErrorDetailsLengthSize +
           kQuicMaxStreamIdSize;
  }
  
  // static
  size_t QuicFramer::GetWindowUpdateFrameSize() {
    return kQuicFrameTypeSize + kQuicMaxStreamIdSize + kQuicMaxStreamOffsetSize;
  }
  
  // static
  size_t QuicFramer::GetBlockedFrameSize() {
    return kQuicFrameTypeSize + kQuicMaxStreamIdSize;
  }
  
  // static
  size_t QuicFramer::GetStreamIdSize(QuicStreamId stream_id) {
    // Sizes are 1 through 4 bytes.
    for (int i = 1; i <= 4; ++i) {
      stream_id >>= 8;
      if (stream_id == 0) {
        return i;
      }
    }
    LOG(DFATAL) << "Failed to determine StreamIDSize.";
    return 4;
  }
  
  // static
  size_t QuicFramer::GetStreamOffsetSize(QuicStreamOffset offset) {
    // 0 is a special case.
    if (offset == 0) {
      return 0;
    }
    // 2 through 8 are the remaining sizes.
    offset >>= 8;
    for (int i = 2; i <= 8; ++i) {
      offset >>= 8;
      if (offset == 0) {
        return i;
      }
    }
    LOG(DFATAL) << "Failed to determine StreamOffsetSize.";
    return 8;
  }
  
  // static
  size_t QuicFramer::GetVersionNegotiationPacketSize(size_t number_versions) {
    return kPublicFlagsSize + PACKET_8BYTE_CONNECTION_ID +
           number_versions * kQuicVersionSize;
  }
  
  bool QuicFramer::IsSupportedVersion(const QuicVersion version) const {
    for (size_t i = 0; i < supported_versions_.size(); ++i) {
      if (version == supported_versions_[i]) {
        return true;
      }
    }
    return false;
  }
  
  size_t QuicFramer::GetSerializedFrameLength(
      const QuicFrame& frame,
      size_t free_bytes,
      bool first_frame,
      bool last_frame,
      InFecGroup is_in_fec_group,
      QuicPacketNumberLength packet_number_length) {
    // Prevent a rare crash reported in b/19458523.
    if ((frame.type == STREAM_FRAME || frame.type == ACK_FRAME) &&
        frame.stream_frame == nullptr) {
      LOG(DFATAL) << "Cannot compute the length of a null frame. "
                  << "type:" << frame.type << "free_bytes:" << free_bytes
                  << " first_frame:" << first_frame
                  << " last_frame:" << last_frame
                  << " is_in_fec:" << is_in_fec_group
                  << " seq num length:" << packet_number_length;
      set_error(QUIC_INTERNAL_ERROR);
      visitor_->OnError(this);
      return 0;
    }
    if (frame.type == PADDING_FRAME) {
      // PADDING implies end of packet.
      return free_bytes;
    }
    size_t frame_len = ComputeFrameLength(frame, last_frame, is_in_fec_group,
                                          packet_number_length);
    if (frame_len <= free_bytes) {
      // Frame fits within packet. Note that acks may be truncated.
      return frame_len;
    }
    // Only truncate the first frame in a packet, so if subsequent ones go
    // over, stop including more frames.
    if (!first_frame) {
      return 0;
    }
    bool can_truncate =
        frame.type == ACK_FRAME &&
        free_bytes >= GetMinAckFrameSize(PACKET_6BYTE_PACKET_NUMBER);
    if (can_truncate) {
      // Truncate the frame so the packet will not exceed kMaxPacketSize.
      // Note that we may not use every byte of the writer in this case.
      DVLOG(1) << "Truncating large frame, free bytes: " << free_bytes;
      return free_bytes;
    }
    return 0;
  }
  
  QuicFramer::AckFrameInfo::AckFrameInfo() : max_delta(0) {}
  
  QuicFramer::AckFrameInfo::~AckFrameInfo() {}
  
  // static
  QuicPacketEntropyHash QuicFramer::GetPacketEntropyHash(
      const QuicPacketHeader& header) {
    return header.entropy_flag << (header.packet_number % 8);
  }
  
  size_t QuicFramer::BuildDataPacket(const QuicPacketHeader& header,
                                     const QuicFrames& frames,
                                     char* buffer,
                                     size_t packet_length) {
    QuicDataWriter writer(packet_length, buffer);
    if (!AppendPacketHeader(header, &writer)) {
      LOG(DFATAL) << "AppendPacketHeader failed";
      return 0;
    }
  
    size_t i = 0;
    for (const QuicFrame& frame : frames) {
      // Determine if we should write stream frame length in header.
      const bool no_stream_frame_length =
          (header.is_in_fec_group == NOT_IN_FEC_GROUP) &&
          (i == frames.size() - 1);
      if (!AppendTypeByte(frame, no_stream_frame_length, &writer)) {
        LOG(DFATAL) << "AppendTypeByte failed";
        return 0;
      }
  
      switch (frame.type) {
        case PADDING_FRAME:
          writer.WritePadding();
          break;
        case STREAM_FRAME:
          if (!AppendStreamFrame(*frame.stream_frame, no_stream_frame_length,
                                 &writer)) {
            LOG(DFATAL) << "AppendStreamFrame failed";
            return 0;
          }
          break;
        case ACK_FRAME:
          if (!AppendAckFrameAndTypeByte(header, *frame.ack_frame, &writer)) {
            LOG(DFATAL) << "AppendAckFrameAndTypeByte failed";
            return 0;
          }
          break;
        case STOP_WAITING_FRAME:
          if (!AppendStopWaitingFrame(header, *frame.stop_waiting_frame,
                                      &writer)) {
            LOG(DFATAL) << "AppendStopWaitingFrame failed";
            return 0;
          }
          break;
        case MTU_DISCOVERY_FRAME:
        // MTU discovery frames are serialized as ping frames.
        case PING_FRAME:
          // Ping has no payload.
          break;
        case RST_STREAM_FRAME:
          if (!AppendRstStreamFrame(*frame.rst_stream_frame, &writer)) {
            LOG(DFATAL) << "AppendRstStreamFrame failed";
            return 0;
          }
          break;
        case CONNECTION_CLOSE_FRAME:
          if (!AppendConnectionCloseFrame(*frame.connection_close_frame,
                                          &writer)) {
            LOG(DFATAL) << "AppendConnectionCloseFrame failed";
            return 0;
          }
          break;
        case GOAWAY_FRAME:
          if (!AppendGoAwayFrame(*frame.goaway_frame, &writer)) {
            LOG(DFATAL) << "AppendGoAwayFrame failed";
            return 0;
          }
          break;
        case WINDOW_UPDATE_FRAME:
          if (!AppendWindowUpdateFrame(*frame.window_update_frame, &writer)) {
            LOG(DFATAL) << "AppendWindowUpdateFrame failed";
            return 0;
          }
          break;
        case BLOCKED_FRAME:
          if (!AppendBlockedFrame(*frame.blocked_frame, &writer)) {
            LOG(DFATAL) << "AppendBlockedFrame failed";
            return 0;
          }
          break;
        default:
          RaiseError(QUIC_INVALID_FRAME_DATA);
          LOG(DFATAL) << "QUIC_INVALID_FRAME_DATA";
          return 0;
      }
      ++i;
    }
  
    return writer.length();
  }
  
  QuicPacket* QuicFramer::BuildFecPacket(const QuicPacketHeader& header,
                                         StringPiece redundancy) {
    DCHECK_EQ(IN_FEC_GROUP, header.is_in_fec_group);
    DCHECK_NE(0u, header.fec_group);
    size_t len = GetPacketHeaderSize(header);
    len += redundancy.length();
  
    scoped_ptr<char[]> buffer(new char[len]);
    QuicDataWriter writer(len, buffer.get());
    if (!AppendPacketHeader(header, &writer)) {
      LOG(DFATAL) << "AppendPacketHeader failed";
      return nullptr;
    }
  
    if (!writer.WriteBytes(redundancy.data(), redundancy.length())) {
      LOG(DFATAL) << "Failed to add FEC";
      return nullptr;
    }
  
    return new QuicPacket(buffer.release(), len, true,
                          header.public_header.connection_id_length,
                          header.public_header.version_flag,
                          header.public_header.packet_number_length);
  }
  
  // static
  QuicEncryptedPacket* QuicFramer::BuildPublicResetPacket(
      const QuicPublicResetPacket& packet) {
    DCHECK(packet.public_header.reset_flag);
  
    CryptoHandshakeMessage reset;
    reset.set_tag(kPRST);
    reset.SetValue(kRNON, packet.nonce_proof);
    reset.SetValue(kRSEQ, packet.rejected_packet_number);
    if (!packet.client_address.address().empty()) {
      // packet.client_address is non-empty.
      QuicSocketAddressCoder address_coder(packet.client_address);
      string serialized_address = address_coder.Encode();
      if (serialized_address.empty()) {
        return nullptr;
      }
      reset.SetStringPiece(kCADR, serialized_address);
    }
    const QuicData& reset_serialized = reset.GetSerialized();
  
    size_t len =
        kPublicFlagsSize + PACKET_8BYTE_CONNECTION_ID + reset_serialized.length();
    scoped_ptr<char[]> buffer(new char[len]);
    QuicDataWriter writer(len, buffer.get());
  
    uint8_t flags = static_cast<uint8_t>(PACKET_PUBLIC_FLAGS_RST |
                                         PACKET_PUBLIC_FLAGS_8BYTE_CONNECTION_ID);
    if (!writer.WriteUInt8(flags)) {
      return nullptr;
    }
  
    if (!writer.WriteUInt64(packet.public_header.connection_id)) {
      return nullptr;
    }
  
    if (!writer.WriteBytes(reset_serialized.data(), reset_serialized.length())) {
      return nullptr;
    }
  
    return new QuicEncryptedPacket(buffer.release(), len, true);
  }
  
  // static
  QuicEncryptedPacket* QuicFramer::BuildVersionNegotiationPacket(
      QuicConnectionId connection_id,
      const QuicVersionVector& versions) {
    DCHECK(!versions.empty());
    size_t len = GetVersionNegotiationPacketSize(versions.size());
    scoped_ptr<char[]> buffer(new char[len]);
    QuicDataWriter writer(len, buffer.get());
  
    uint8_t flags = static_cast<uint8_t>(PACKET_PUBLIC_FLAGS_VERSION |
                                         PACKET_PUBLIC_FLAGS_8BYTE_CONNECTION_ID);
    if (!writer.WriteUInt8(flags)) {
      return nullptr;
    }
  
    if (!writer.WriteUInt64(connection_id)) {
      return nullptr;
    }
  
    for (QuicVersion version : versions) {
      if (!writer.WriteUInt32(QuicVersionToQuicTag(version))) {
        return nullptr;
      }
    }
  
    return new QuicEncryptedPacket(buffer.release(), len, true);
  }
  
  bool QuicFramer::ProcessPacket(const QuicEncryptedPacket& packet) {
    QuicDataReader reader(packet.data(), packet.length());
  
    visitor_->OnPacket();
  
    // First parse the public header.
    QuicPacketPublicHeader public_header;
    if (!ProcessPublicHeader(&reader, &public_header)) {
      DLOG(WARNING) << "Unable to process public header.";
      DCHECK_NE("", detailed_error_);
      return RaiseError(QUIC_INVALID_PACKET_HEADER);
    }
  
    if (!visitor_->OnUnauthenticatedPublicHeader(public_header)) {
      // The visitor suppresses further processing of the packet.
      return true;
    }
  
    if (perspective_ == Perspective::IS_SERVER && public_header.version_flag &&
        public_header.versions[0] != quic_version_) {
      if (!visitor_->OnProtocolVersionMismatch(public_header.versions[0])) {
        return true;
      }
    }
  
    bool rv;
    if (perspective_ == Perspective::IS_CLIENT && public_header.version_flag) {
      rv = ProcessVersionNegotiationPacket(&reader, &public_header);
    } else if (public_header.reset_flag) {
      rv = ProcessPublicResetPacket(&reader, public_header);
    } else if (packet.length() <= kMaxPacketSize) {
      // The optimized decryption algorithm implementations run faster when
      // operating on aligned memory.
      //
      // TODO(rtenneti): Change the default 64 alignas value (used the default
      // value from CACHELINE_SIZE).
      ALIGNAS(64) char buffer[kMaxPacketSize];
      rv = ProcessDataPacket(&reader, public_header, packet, buffer,
                             kMaxPacketSize);
    } else {
      scoped_ptr<char[]> large_buffer(new char[packet.length()]);
      rv = ProcessDataPacket(&reader, public_header, packet, large_buffer.get(),
                             packet.length());
      LOG_IF(DFATAL, rv) << "QUIC should never successfully process packets "
                         << "larger than kMaxPacketSize. packet size:"
                         << packet.length();
    }
  
    return rv;
  }
  
  bool QuicFramer::ProcessVersionNegotiationPacket(
      QuicDataReader* reader,
      QuicPacketPublicHeader* public_header) {
    DCHECK_EQ(Perspective::IS_CLIENT, perspective_);
    // Try reading at least once to raise error if the packet is invalid.
    do {
      QuicTag version;
      if (!reader->ReadBytes(&version, kQuicVersionSize)) {
        set_detailed_error("Unable to read supported version in negotiation.");
        return RaiseError(QUIC_INVALID_VERSION_NEGOTIATION_PACKET);
      }
      public_header->versions.push_back(QuicTagToQuicVersion(version));
    } while (!reader->IsDoneReading());
  
    visitor_->OnVersionNegotiationPacket(*public_header);
    return true;
  }
  
  bool QuicFramer::ProcessDataPacket(QuicDataReader* encrypted_reader,
                                     const QuicPacketPublicHeader& public_header,
                                     const QuicEncryptedPacket& packet,
                                     char* decrypted_buffer,
                                     size_t buffer_length) {
    QuicPacketHeader header(public_header);
    if (!ProcessUnauthenticatedHeader(encrypted_reader, &header)) {
      DLOG(WARNING) << "Unable to process packet header.  Stopping parsing.";
      return false;
    }
  
    size_t decrypted_length = 0;
    if (!DecryptPayload(encrypted_reader, header, packet, decrypted_buffer,
                        buffer_length, &decrypted_length)) {
      set_detailed_error("Unable to decrypt payload.");
      return RaiseError(QUIC_DECRYPTION_FAILURE);
    }
  
    QuicDataReader reader(decrypted_buffer, decrypted_length);
    if (!ProcessAuthenticatedHeader(&reader, &header)) {
      DLOG(WARNING) << "Unable to process packet header.  Stopping parsing.";
      return false;
    }
  
    if (!visitor_->OnPacketHeader(header)) {
      // The visitor suppresses further processing of the packet.
      return true;
    }
  
    if (packet.length() > kMaxPacketSize) {
      // If the packet has gotten this far, it should not be too large.
      LOG(DFATAL) << "Packet too large:" << packet.length();
      return RaiseError(QUIC_PACKET_TOO_LARGE);
    }
  
    // Handle the payload.
    if (!header.fec_flag) {
      if (header.is_in_fec_group == IN_FEC_GROUP) {
        StringPiece payload = reader.PeekRemainingPayload();
        visitor_->OnFecProtectedPayload(payload);
      }
      if (!ProcessFrameData(&reader, header)) {
        DCHECK_NE(QUIC_NO_ERROR, error_);  // ProcessFrameData sets the error.
        DLOG(WARNING) << "Unable to process frame data.";
        return false;
      }
    } else {
      visitor_->OnFecData(reader.ReadRemainingPayload());
    }
  
    visitor_->OnPacketComplete();
    return true;
  }
  
  bool QuicFramer::ProcessPublicResetPacket(
      QuicDataReader* reader,
      const QuicPacketPublicHeader& public_header) {
    QuicPublicResetPacket packet(public_header);
  
    scoped_ptr<CryptoHandshakeMessage> reset(
        CryptoFramer::ParseMessage(reader->ReadRemainingPayload()));
    if (!reset.get()) {
      set_detailed_error("Unable to read reset message.");
      return RaiseError(QUIC_INVALID_PUBLIC_RST_PACKET);
    }
    if (reset->tag() != kPRST) {
      set_detailed_error("Incorrect message tag.");
      return RaiseError(QUIC_INVALID_PUBLIC_RST_PACKET);
    }
  
    if (reset->GetUint64(kRNON, &packet.nonce_proof) != QUIC_NO_ERROR) {
      set_detailed_error("Unable to read nonce proof.");
      return RaiseError(QUIC_INVALID_PUBLIC_RST_PACKET);
    }
    // TODO(satyamshekhar): validate nonce to protect against DoS.
  
    if (reset->GetUint64(kRSEQ, &packet.rejected_packet_number) !=
        QUIC_NO_ERROR) {
      set_detailed_error("Unable to read rejected packet number.");
      return RaiseError(QUIC_INVALID_PUBLIC_RST_PACKET);
    }
  
    StringPiece address;
    if (reset->GetStringPiece(kCADR, &address)) {
      QuicSocketAddressCoder address_coder;
      if (address_coder.Decode(address.data(), address.length())) {
        packet.client_address =
            IPEndPoint(address_coder.ip(), address_coder.port());
      }
    }
  
    visitor_->OnPublicResetPacket(packet);
    return true;
  }
  
  bool QuicFramer::ProcessRevivedPacket(QuicPacketHeader* header,
                                        StringPiece payload) {
    visitor_->OnRevivedPacket();
  
    header->entropy_hash = GetPacketEntropyHash(*header);
  
    if (!visitor_->OnPacketHeader(*header)) {
      return true;
    }
  
    if (payload.length() > kMaxPacketSize) {
      set_detailed_error("Revived packet too large.");
      return RaiseError(QUIC_PACKET_TOO_LARGE);
    }
  
    QuicDataReader reader(payload.data(), payload.length());
    if (!ProcessFrameData(&reader, *header)) {
      DCHECK_NE(QUIC_NO_ERROR, error_);  // ProcessFrameData sets the error.
      DLOG(WARNING) << "Unable to process frame data.";
      return false;
    }
  
    visitor_->OnPacketComplete();
    return true;
  }
  
  bool QuicFramer::AppendPacketHeader(const QuicPacketHeader& header,
                                      QuicDataWriter* writer) {
    DVLOG(1) << "Appending header: " << header;
    DCHECK(header.fec_group > 0 || header.is_in_fec_group == NOT_IN_FEC_GROUP);
    uint8_t public_flags = 0;
    if (header.public_header.reset_flag) {
      public_flags |= PACKET_PUBLIC_FLAGS_RST;
    }
    if (header.public_header.version_flag) {
      public_flags |= PACKET_PUBLIC_FLAGS_VERSION;
    }
    if (header.public_header.multipath_flag) {
      public_flags |= PACKET_PUBLIC_FLAGS_MULTIPATH;
    }
  
    public_flags |=
        GetSequenceNumberFlags(header.public_header.packet_number_length)
        << kPublicHeaderSequenceNumberShift;
  
    switch (header.public_header.connection_id_length) {
      case PACKET_0BYTE_CONNECTION_ID:
        if (!writer->WriteUInt8(public_flags |
                                PACKET_PUBLIC_FLAGS_0BYTE_CONNECTION_ID)) {
          return false;
        }
        break;
      case PACKET_1BYTE_CONNECTION_ID:
        if (!writer->WriteUInt8(public_flags |
                                PACKET_PUBLIC_FLAGS_1BYTE_CONNECTION_ID)) {
          return false;
        }
        if (!writer->WriteUInt8(header.public_header.connection_id &
                                k1ByteConnectionIdMask)) {
          return false;
        }
        break;
      case PACKET_4BYTE_CONNECTION_ID:
        if (!writer->WriteUInt8(public_flags |
                                PACKET_PUBLIC_FLAGS_4BYTE_CONNECTION_ID)) {
          return false;
        }
        if (!writer->WriteUInt32(header.public_header.connection_id &
                                 k4ByteConnectionIdMask)) {
          return false;
        }
        break;
      case PACKET_8BYTE_CONNECTION_ID:
        if (!writer->WriteUInt8(public_flags |
                                PACKET_PUBLIC_FLAGS_8BYTE_CONNECTION_ID)) {
          return false;
        }
        if (!writer->WriteUInt64(header.public_header.connection_id)) {
          return false;
        }
        break;
    }
    last_serialized_connection_id_ = header.public_header.connection_id;
  
    if (header.public_header.version_flag) {
      DCHECK_EQ(Perspective::IS_CLIENT, perspective_);
      QuicTag tag = QuicVersionToQuicTag(quic_version_);
      writer->WriteUInt32(tag);
      DVLOG(1) << "version = " << quic_version_ << ", tag = '"
               << QuicUtils::TagToString(tag) << "'";
    }
  
    if (header.public_header.multipath_flag &&
        !writer->WriteUInt8(header.path_id)) {
      return false;
    }
  
    if (!AppendPacketSequenceNumber(header.public_header.packet_number_length,
                                    header.packet_number, writer)) {
      return false;
    }
  
    uint8_t private_flags = 0;
    if (header.entropy_flag) {
      private_flags |= PACKET_PRIVATE_FLAGS_ENTROPY;
    }
    if (header.is_in_fec_group == IN_FEC_GROUP) {
      private_flags |= PACKET_PRIVATE_FLAGS_FEC_GROUP;
    }
    if (header.fec_flag) {
      private_flags |= PACKET_PRIVATE_FLAGS_FEC;
    }
    if (!writer->WriteUInt8(private_flags)) {
      return false;
    }
  
    // The FEC group number is the packet number of the first fec
    // protected packet, or 0 if this packet is not protected.
    if (header.is_in_fec_group == IN_FEC_GROUP) {
      DCHECK_LE(header.fec_group, header.packet_number);
      DCHECK_LT(header.packet_number - header.fec_group, 255u);
      // Offset from the current packet number to the first fec
      // protected packet.
      uint8_t first_fec_protected_packet_offset =
          static_cast<uint8_t>(header.packet_number - header.fec_group);
      if (!writer->WriteBytes(&first_fec_protected_packet_offset, 1)) {
        return false;
      }
    }
  
    return true;
  }
  
  const QuicTime::Delta QuicFramer::CalculateTimestampFromWire(
      uint32_t time_delta_us) {
    // The new time_delta might have wrapped to the next epoch, or it
    // might have reverse wrapped to the previous epoch, or it might
    // remain in the same epoch. Select the time closest to the previous
    // time.
    //
    // epoch_delta is the delta between epochs. A delta is 4 bytes of
    // microseconds.
    const uint64_t epoch_delta = UINT64_C(1) << 32;
    uint64_t epoch = last_timestamp_.ToMicroseconds() & ~(epoch_delta - 1);
    // Wrapping is safe here because a wrapped value will not be ClosestTo below.
    uint64_t prev_epoch = epoch - epoch_delta;
    uint64_t next_epoch = epoch + epoch_delta;
  
    uint64_t time = ClosestTo(
        last_timestamp_.ToMicroseconds(), epoch + time_delta_us,
        ClosestTo(last_timestamp_.ToMicroseconds(), prev_epoch + time_delta_us,
                  next_epoch + time_delta_us));
  
    return QuicTime::Delta::FromMicroseconds(time);
  }
  
  bool QuicFramer::IsValidPath(QuicPathId path_id,
                               QuicPacketNumber* last_packet_number) {
    if (ContainsKey(closed_paths_, path_id)) {
      // Path is closed.
      return false;
    }
  
    if (path_id == last_path_id_) {
      *last_packet_number = last_packet_number_;
      return true;
    }
  
    if (ContainsKey(last_packet_numbers_, path_id)) {
      *last_packet_number = last_packet_numbers_[path_id];
    } else {
      *last_packet_number = 0;
    }
  
    return true;
  }
  
  void QuicFramer::OnPathClosed(QuicPathId path_id) {
    closed_paths_.insert(path_id);
    last_packet_numbers_.erase(path_id);
  }
  
  QuicPacketNumber QuicFramer::CalculatePacketNumberFromWire(
      QuicPacketNumberLength packet_number_length,
      QuicPacketNumber last_packet_number,
      QuicPacketNumber packet_number) const {
    // The new packet number might have wrapped to the next epoch, or
    // it might have reverse wrapped to the previous epoch, or it might
    // remain in the same epoch.  Select the packet number closest to the
    // next expected packet number, the previous packet number plus 1.
  
    // epoch_delta is the delta between epochs the packet number was serialized
    // with, so the correct value is likely the same epoch as the last sequence
    // number or an adjacent epoch.
    const QuicPacketNumber epoch_delta = UINT64_C(1)
                                         << (8 * packet_number_length);
    QuicPacketNumber next_packet_number = last_packet_number + 1;
    QuicPacketNumber epoch = last_packet_number & ~(epoch_delta - 1);
    QuicPacketNumber prev_epoch = epoch - epoch_delta;
    QuicPacketNumber next_epoch = epoch + epoch_delta;
  
    return ClosestTo(next_packet_number, epoch + packet_number,
                     ClosestTo(next_packet_number, prev_epoch + packet_number,
                               next_epoch + packet_number));
  }
  
  bool QuicFramer::ProcessPublicHeader(QuicDataReader* reader,
                                       QuicPacketPublicHeader* public_header) {
    uint8_t public_flags;
    if (!reader->ReadBytes(&public_flags, 1)) {
      set_detailed_error("Unable to read public flags.");
      return false;
    }
  
    public_header->multipath_flag =
        (public_flags & PACKET_PUBLIC_FLAGS_MULTIPATH) != 0;
    public_header->reset_flag = (public_flags & PACKET_PUBLIC_FLAGS_RST) != 0;
    public_header->version_flag =
        (public_flags & PACKET_PUBLIC_FLAGS_VERSION) != 0;
  
    if (validate_flags_ && !public_header->version_flag &&
        public_flags > PACKET_PUBLIC_FLAGS_MAX) {
      set_detailed_error("Illegal public flags value.");
      return false;
    }
  
    if (public_header->reset_flag && public_header->version_flag) {
      set_detailed_error("Got version flag in reset packet");
      return false;
    }
  
    switch (public_flags & PACKET_PUBLIC_FLAGS_8BYTE_CONNECTION_ID) {
      case PACKET_PUBLIC_FLAGS_8BYTE_CONNECTION_ID:
        if (!reader->ReadUInt64(&public_header->connection_id)) {
          set_detailed_error("Unable to read ConnectionId.");
          return false;
        }
        public_header->connection_id_length = PACKET_8BYTE_CONNECTION_ID;
        break;
      case PACKET_PUBLIC_FLAGS_4BYTE_CONNECTION_ID:
        // If the connection_id is truncated, expect to read the last serialized
        // connection_id.
        if (!reader->ReadBytes(&public_header->connection_id,
                               PACKET_4BYTE_CONNECTION_ID)) {
          set_detailed_error("Unable to read ConnectionId.");
          return false;
        }
        if (last_serialized_connection_id_ &&
            (public_header->connection_id & k4ByteConnectionIdMask) !=
                (last_serialized_connection_id_ & k4ByteConnectionIdMask)) {
          set_detailed_error(
              "Truncated 4 byte ConnectionId does not match "
              "previous connection_id.");
          return false;
        }
        public_header->connection_id_length = PACKET_4BYTE_CONNECTION_ID;
        public_header->connection_id = last_serialized_connection_id_;
        break;
      case PACKET_PUBLIC_FLAGS_1BYTE_CONNECTION_ID:
        if (!reader->ReadBytes(&public_header->connection_id,
                               PACKET_1BYTE_CONNECTION_ID)) {
          set_detailed_error("Unable to read ConnectionId.");
          return false;
        }
        if (last_serialized_connection_id_ &&
            (public_header->connection_id & k1ByteConnectionIdMask) !=
                (last_serialized_connection_id_ & k1ByteConnectionIdMask)) {
          set_detailed_error(
              "Truncated 1 byte ConnectionId does not match "
              "previous connection_id.");
          return false;
        }
        public_header->connection_id_length = PACKET_1BYTE_CONNECTION_ID;
        public_header->connection_id = last_serialized_connection_id_;
        break;
      case PACKET_PUBLIC_FLAGS_0BYTE_CONNECTION_ID:
        public_header->connection_id_length = PACKET_0BYTE_CONNECTION_ID;
        public_header->connection_id = last_serialized_connection_id_;
        break;
    }
  
    public_header->packet_number_length = ReadSequenceNumberLength(
        public_flags >> kPublicHeaderSequenceNumberShift);
  
    // Read the version only if the packet is from the client.
    // version flag from the server means version negotiation packet.
    if (public_header->version_flag && perspective_ == Perspective::IS_SERVER) {
      QuicTag version_tag;
      if (!reader->ReadUInt32(&version_tag)) {
        set_detailed_error("Unable to read protocol version.");
        return false;
      }
  
      // If the version from the new packet is the same as the version of this
      // framer, then the public flags should be set to something we understand.
      // If not, this raises an error.
      QuicVersion version = QuicTagToQuicVersion(version_tag);
      if (version == quic_version_ && public_flags > PACKET_PUBLIC_FLAGS_MAX) {
        set_detailed_error("Illegal public flags value.");
        return false;
      }
      public_header->versions.push_back(version);
    }
    return true;
  }
  
  // static
  QuicPacketNumberLength QuicFramer::GetMinSequenceNumberLength(
      QuicPacketNumber packet_number) {
    if (packet_number < 1 << (PACKET_1BYTE_PACKET_NUMBER * 8)) {
      return PACKET_1BYTE_PACKET_NUMBER;
    } else if (packet_number < 1 << (PACKET_2BYTE_PACKET_NUMBER * 8)) {
      return PACKET_2BYTE_PACKET_NUMBER;
    } else if (packet_number < UINT64_C(1) << (PACKET_4BYTE_PACKET_NUMBER * 8)) {
      return PACKET_4BYTE_PACKET_NUMBER;
    } else {
      return PACKET_6BYTE_PACKET_NUMBER;
    }
  }
  
  // static
  uint8_t QuicFramer::GetSequenceNumberFlags(
      QuicPacketNumberLength packet_number_length) {
    switch (packet_number_length) {
      case PACKET_1BYTE_PACKET_NUMBER:
        return PACKET_FLAGS_1BYTE_PACKET;
      case PACKET_2BYTE_PACKET_NUMBER:
        return PACKET_FLAGS_2BYTE_PACKET;
      case PACKET_4BYTE_PACKET_NUMBER:
        return PACKET_FLAGS_4BYTE_PACKET;
      case PACKET_6BYTE_PACKET_NUMBER:
        return PACKET_FLAGS_6BYTE_PACKET;
      default:
        LOG(DFATAL) << "Unreachable case statement.";
        return PACKET_FLAGS_6BYTE_PACKET;
    }
  }
  
  // static
  QuicFramer::AckFrameInfo QuicFramer::GetAckFrameInfo(
      const QuicAckFrame& frame) {
    AckFrameInfo ack_info;
    if (frame.missing_packets.Empty()) {
      return ack_info;
    }
    DCHECK_GE(frame.largest_observed, frame.missing_packets.Max());
    size_t cur_range_length = 0;
    PacketNumberQueue::const_iterator iter = frame.missing_packets.begin();
    // TODO(jdorfman): Switch this logic to use the intervals in PacketNumberQueue
    // instead of reconstructing them from the sequence.
    QuicPacketNumber last_missing = *iter;
    ++iter;
    for (; iter != frame.missing_packets.end(); ++iter) {
      if (cur_range_length < numeric_limits<uint8_t>::max() &&
          *iter == (last_missing + 1)) {
        ++cur_range_length;
      } else {
        ack_info.nack_ranges[last_missing - cur_range_length] =
            static_cast<uint8_t>(cur_range_length);
        cur_range_length = 0;
      }
      ack_info.max_delta = max(ack_info.max_delta, *iter - last_missing);
      last_missing = *iter;
    }
    // Include the last nack range.
    ack_info.nack_ranges[last_missing - cur_range_length] =
        static_cast<uint8_t>(cur_range_length);
    // Include the range to the largest observed.
    ack_info.max_delta =
        max(ack_info.max_delta, frame.largest_observed - last_missing);
    return ack_info;
  }
  
  bool QuicFramer::ProcessUnauthenticatedHeader(QuicDataReader* encrypted_reader,
                                                QuicPacketHeader* header) {
    if (header->public_header.multipath_flag &&
        !ProcessPathId(encrypted_reader, &header->path_id)) {
      set_detailed_error("Unable to read path id.");
      return RaiseError(QUIC_INVALID_PACKET_HEADER);
    }
  
    QuicPacketNumber last_packet_number = last_packet_number_;
    if (header->public_header.multipath_flag &&
        !IsValidPath(header->path_id, &last_packet_number)) {
      // Stop processing because path is closed.
      return false;
    }
  
    if (!ProcessPacketSequenceNumber(
            encrypted_reader, header->public_header.packet_number_length,
            last_packet_number, &header->packet_number)) {
      set_detailed_error("Unable to read packet number.");
      return RaiseError(QUIC_INVALID_PACKET_HEADER);
    }
  
    if (header->packet_number == 0u) {
      set_detailed_error("packet numbers cannot be 0.");
      return RaiseError(QUIC_INVALID_PACKET_HEADER);
    }
  
    if (!visitor_->OnUnauthenticatedHeader(*header)) {
      return false;
    }
    return true;
  }
  
  bool QuicFramer::ProcessAuthenticatedHeader(QuicDataReader* reader,
                                              QuicPacketHeader* header) {
    uint8_t private_flags;
    if (!reader->ReadBytes(&private_flags, 1)) {
      set_detailed_error("Unable to read private flags.");
      return RaiseError(QUIC_INVALID_PACKET_HEADER);
    }
  
    if (private_flags > PACKET_PRIVATE_FLAGS_MAX) {
      set_detailed_error("Illegal private flags value.");
      return RaiseError(QUIC_INVALID_PACKET_HEADER);
    }
  
    header->entropy_flag = (private_flags & PACKET_PRIVATE_FLAGS_ENTROPY) != 0;
    header->fec_flag = (private_flags & PACKET_PRIVATE_FLAGS_FEC) != 0;
  
    if ((private_flags & PACKET_PRIVATE_FLAGS_FEC_GROUP) != 0) {
      header->is_in_fec_group = IN_FEC_GROUP;
      uint8_t first_fec_protected_packet_offset;
      if (!reader->ReadBytes(&first_fec_protected_packet_offset, 1)) {
        set_detailed_error("Unable to read first fec protected packet offset.");
        return RaiseError(QUIC_INVALID_PACKET_HEADER);
      }
      if (first_fec_protected_packet_offset >= header->packet_number) {
        set_detailed_error(
            "First fec protected packet offset must be less "
            "than the packet number.");
        return RaiseError(QUIC_INVALID_PACKET_HEADER);
      }
      header->fec_group =
          header->packet_number - first_fec_protected_packet_offset;
    }
  
    header->entropy_hash = GetPacketEntropyHash(*header);
    // Set the last packet number after we have decrypted the packet
    // so we are confident is not attacker controlled.
    if (header->public_header.multipath_flag &&
        header->path_id != last_path_id_) {
      if (last_path_id_ != kInvalidPathId) {
        // Save current last packet number before changing path.
        last_packet_numbers_[last_path_id_] = last_packet_number_;
      }
      // Change path.
      last_path_id_ = header->path_id;
    }
    last_packet_number_ = header->packet_number;
    return true;
  }
  
  bool QuicFramer::ProcessPathId(QuicDataReader* reader, QuicPathId* path_id) {
    if (!reader->ReadBytes(path_id, 1)) {
      return false;
    }
  
    return true;
  }
  
  bool QuicFramer::ProcessPacketSequenceNumber(
      QuicDataReader* reader,
      QuicPacketNumberLength packet_number_length,
      QuicPacketNumber last_packet_number,
      QuicPacketNumber* packet_number) {
    QuicPacketNumber wire_packet_number = 0u;
    if (!reader->ReadBytes(&wire_packet_number, packet_number_length)) {
      return false;
    }
  
    // TODO(ianswett): Explore the usefulness of trying multiple packet numbers
    // in case the first guess is incorrect.
    *packet_number = CalculatePacketNumberFromWire(
        packet_number_length, last_packet_number, wire_packet_number);
    return true;
  }
  
  bool QuicFramer::ProcessFrameData(QuicDataReader* reader,
                                    const QuicPacketHeader& header) {
    if (reader->IsDoneReading()) {
      set_detailed_error("Packet has no frames.");
      return RaiseError(QUIC_MISSING_PAYLOAD);
    }
    while (!reader->IsDoneReading()) {
      uint8_t frame_type;
      if (!reader->ReadBytes(&frame_type, 1)) {
        set_detailed_error("Unable to read frame type.");
        return RaiseError(QUIC_INVALID_FRAME_DATA);
      }
  
      if (frame_type & kQuicFrameTypeSpecialMask) {
        // Stream Frame
        if (frame_type & kQuicFrameTypeStreamMask) {
          QuicStreamFrame frame;
          if (!ProcessStreamFrame(reader, frame_type, &frame)) {
            return RaiseError(QUIC_INVALID_STREAM_DATA);
          }
          if (!visitor_->OnStreamFrame(frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        // Ack Frame
        if (frame_type & kQuicFrameTypeAckMask) {
          QuicAckFrame frame;
          if (!ProcessAckFrame(reader, frame_type, &frame)) {
            return RaiseError(QUIC_INVALID_ACK_DATA);
          }
          if (!visitor_->OnAckFrame(frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        // This was a special frame type that did not match any
        // of the known ones. Error.
        set_detailed_error("Illegal frame type.");
        DLOG(WARNING) << "Illegal frame type: " << static_cast<int>(frame_type);
        return RaiseError(QUIC_INVALID_FRAME_DATA);
      }
  
      switch (frame_type) {
        case PADDING_FRAME:
          // We're done with the packet.
          return true;
  
        case RST_STREAM_FRAME: {
          QuicRstStreamFrame frame;
          if (!ProcessRstStreamFrame(reader, &frame)) {
            return RaiseError(QUIC_INVALID_RST_STREAM_DATA);
          }
          if (!visitor_->OnRstStreamFrame(frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        case CONNECTION_CLOSE_FRAME: {
          QuicConnectionCloseFrame frame;
          if (!ProcessConnectionCloseFrame(reader, &frame)) {
            return RaiseError(QUIC_INVALID_CONNECTION_CLOSE_DATA);
          }
  
          if (!visitor_->OnConnectionCloseFrame(frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        case GOAWAY_FRAME: {
          QuicGoAwayFrame goaway_frame;
          if (!ProcessGoAwayFrame(reader, &goaway_frame)) {
            return RaiseError(QUIC_INVALID_GOAWAY_DATA);
          }
          if (!visitor_->OnGoAwayFrame(goaway_frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        case WINDOW_UPDATE_FRAME: {
          QuicWindowUpdateFrame window_update_frame;
          if (!ProcessWindowUpdateFrame(reader, &window_update_frame)) {
            return RaiseError(QUIC_INVALID_WINDOW_UPDATE_DATA);
          }
          if (!visitor_->OnWindowUpdateFrame(window_update_frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        case BLOCKED_FRAME: {
          QuicBlockedFrame blocked_frame;
          if (!ProcessBlockedFrame(reader, &blocked_frame)) {
            return RaiseError(QUIC_INVALID_BLOCKED_DATA);
          }
          if (!visitor_->OnBlockedFrame(blocked_frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        case STOP_WAITING_FRAME: {
          QuicStopWaitingFrame stop_waiting_frame;
          if (!ProcessStopWaitingFrame(reader, header, &stop_waiting_frame)) {
            return RaiseError(QUIC_INVALID_STOP_WAITING_DATA);
          }
          if (!visitor_->OnStopWaitingFrame(stop_waiting_frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
        case PING_FRAME: {
          // Ping has no payload.
          QuicPingFrame ping_frame;
          if (!visitor_->OnPingFrame(ping_frame)) {
            DVLOG(1) << "Visitor asked to stop further processing.";
            // Returning true since there was no parsing error.
            return true;
          }
          continue;
        }
  
        default:
          set_detailed_error("Illegal frame type.");
          DLOG(WARNING) << "Illegal frame type: " << static_cast<int>(frame_type);
          return RaiseError(QUIC_INVALID_FRAME_DATA);
      }
    }
  
    return true;
  }
  
  bool QuicFramer::ProcessStreamFrame(QuicDataReader* reader,
                                      uint8_t frame_type,
                                      QuicStreamFrame* frame) {
    uint8_t stream_flags = frame_type;
  
    stream_flags &= ~kQuicFrameTypeStreamMask;
  
    // Read from right to left: StreamID, Offset, Data Length, Fin.
    const uint8_t stream_id_length = (stream_flags & kQuicStreamIDLengthMask) + 1;
    stream_flags >>= kQuicStreamIdShift;
  
    uint8_t offset_length = (stream_flags & kQuicStreamOffsetMask);
    // There is no encoding for 1 byte, only 0 and 2 through 8.
    if (offset_length > 0) {
      offset_length += 1;
    }
    stream_flags >>= kQuicStreamOffsetShift;
  
    bool has_data_length =
        (stream_flags & kQuicStreamDataLengthMask) == kQuicStreamDataLengthMask;
    stream_flags >>= kQuicStreamDataLengthShift;
  
    frame->fin = (stream_flags & kQuicStreamFinMask) == kQuicStreamFinShift;
  
    frame->stream_id = 0;
    if (!reader->ReadBytes(&frame->stream_id, stream_id_length)) {
      set_detailed_error("Unable to read stream_id.");
      return false;
    }
  
    frame->offset = 0;
    if (!reader->ReadBytes(&frame->offset, offset_length)) {
      set_detailed_error("Unable to read offset.");
      return false;
    }
  
    // TODO(ianswett): Don't use StringPiece as an intermediary.
    StringPiece data;
    if (has_data_length) {
      if (!reader->ReadStringPiece16(&data)) {
        set_detailed_error("Unable to read frame data.");
        return false;
      }
    } else {
      if (!reader->ReadStringPiece(&data, reader->BytesRemaining())) {
        set_detailed_error("Unable to read frame data.");
        return false;
      }
    }
    frame->frame_buffer = data.data();
    frame->frame_length = static_cast<uint16_t>(data.length());
  
    return true;
  }
  
  bool QuicFramer::ProcessAckFrame(QuicDataReader* reader,
                                   uint8_t frame_type,
                                   QuicAckFrame* ack_frame) {
    // Determine the three lengths from the frame type: largest observed length,
    // missing packet number length, and missing range length.
    const QuicPacketNumberLength missing_packet_number_length =
        ReadSequenceNumberLength(frame_type);
    frame_type >>= kQuicSequenceNumberLengthShift;
    const QuicPacketNumberLength largest_observed_packet_number_length =
        ReadSequenceNumberLength(frame_type);
    frame_type >>= kQuicSequenceNumberLengthShift;
    ack_frame->is_truncated = frame_type & kQuicAckTruncatedMask;
    frame_type >>= kQuicAckTruncatedShift;
    bool has_nacks = frame_type & kQuicHasNacksMask;
  
    if (!reader->ReadBytes(&ack_frame->entropy_hash, 1)) {
      set_detailed_error("Unable to read entropy hash for received packets.");
      return false;
    }
  
    if (!reader->ReadBytes(&ack_frame->largest_observed,
                           largest_observed_packet_number_length)) {
      set_detailed_error("Unable to read largest observed.");
      return false;
    }
  
    uint64_t delta_time_largest_observed_us;
    if (!reader->ReadUFloat16(&delta_time_largest_observed_us)) {
      set_detailed_error("Unable to read delta time largest observed.");
      return false;
    }
  
    if (delta_time_largest_observed_us == kUFloat16MaxValue) {
      ack_frame->delta_time_largest_observed = QuicTime::Delta::Infinite();
    } else {
      ack_frame->delta_time_largest_observed =
          QuicTime::Delta::FromMicroseconds(delta_time_largest_observed_us);
    }
  
    if (!ProcessTimestampsInAckFrame(reader, ack_frame)) {
      return false;
    }
  
    if (!has_nacks) {
      return true;
    }
  
    uint8_t num_missing_ranges;
    if (!reader->ReadBytes(&num_missing_ranges, 1)) {
      set_detailed_error("Unable to read num missing packet ranges.");
      return false;
    }
  
    QuicPacketNumber last_packet_number = ack_frame->largest_observed;
    for (size_t i = 0; i < num_missing_ranges; ++i) {
      QuicPacketNumber missing_delta = 0;
      if (!reader->ReadBytes(&missing_delta, missing_packet_number_length)) {
        set_detailed_error("Unable to read missing packet number delta.");
        return false;
      }
      last_packet_number -= missing_delta;
      QuicPacketNumber range_length = 0;
      if (!reader->ReadBytes(&range_length, PACKET_1BYTE_PACKET_NUMBER)) {
        set_detailed_error("Unable to read missing packet number range.");
        return false;
      }
      ack_frame->missing_packets.Add(last_packet_number - range_length,
                                     last_packet_number + 1);
      // Subtract an extra 1 to ensure ranges are represented efficiently and
      // can't overlap by 1 packet number.  This allows a missing_delta of 0
      // to represent an adjacent nack range.
      last_packet_number -= (range_length + 1);
    }
  
    // Parse the revived packets list.
    // TODO(ianswett): Change the ack frame so it only expresses one revived.
    uint8_t num_revived_packets;
    if (!reader->ReadBytes(&num_revived_packets, 1)) {
      set_detailed_error("Unable to read num revived packets.");
      return false;
    }
  
    for (size_t i = 0; i < num_revived_packets; ++i) {
      QuicPacketNumber revived_packet = 0;
      if (!reader->ReadBytes(&revived_packet,
                             largest_observed_packet_number_length)) {
        set_detailed_error("Unable to read revived packet.");
        return false;
      }
      ack_frame->latest_revived_packet = revived_packet;
    }
  
    return true;
  }
  
  bool QuicFramer::ProcessTimestampsInAckFrame(QuicDataReader* reader,
                                               QuicAckFrame* ack_frame) {
    if (ack_frame->is_truncated) {
      return true;
    }
    uint8_t num_received_packets;
    if (!reader->ReadBytes(&num_received_packets, 1)) {
      set_detailed_error("Unable to read num received packets.");
      return false;
    }
  
    if (num_received_packets > 0) {
      uint8_t delta_from_largest_observed;
      if (!reader->ReadBytes(&delta_from_largest_observed,
                             PACKET_1BYTE_PACKET_NUMBER)) {
        set_detailed_error("Unable to read sequence delta in received packets.");
        return false;
      }
      QuicPacketNumber seq_num =
          ack_frame->largest_observed - delta_from_largest_observed;
  
      // Time delta from the framer creation.
      uint32_t time_delta_us;
      if (!reader->ReadBytes(&time_delta_us, sizeof(time_delta_us))) {
        set_detailed_error("Unable to read time delta in received packets.");
        return false;
      }
  
      last_timestamp_ = CalculateTimestampFromWire(time_delta_us);
  
      ack_frame->received_packet_times.reserve(num_received_packets);
      ack_frame->received_packet_times.push_back(
          std::make_pair(seq_num, creation_time_.Add(last_timestamp_)));
  
      for (uint8_t i = 1; i < num_received_packets; ++i) {
        if (!reader->ReadBytes(&delta_from_largest_observed,
                               PACKET_1BYTE_PACKET_NUMBER)) {
          set_detailed_error(
              "Unable to read sequence delta in received packets.");
          return false;
        }
        seq_num = ack_frame->largest_observed - delta_from_largest_observed;
  
        // Time delta from the previous timestamp.
        uint64_t incremental_time_delta_us;
        if (!reader->ReadUFloat16(&incremental_time_delta_us)) {
          set_detailed_error(
              "Unable to read incremental time delta in received packets.");
          return false;
        }
  
        last_timestamp_ = last_timestamp_.Add(
            QuicTime::Delta::FromMicroseconds(incremental_time_delta_us));
        ack_frame->received_packet_times.push_back(
            std::make_pair(seq_num, creation_time_.Add(last_timestamp_)));
      }
    }
    return true;
  }
  
  bool QuicFramer::ProcessStopWaitingFrame(QuicDataReader* reader,
                                           const QuicPacketHeader& header,
                                           QuicStopWaitingFrame* stop_waiting) {
    if (!reader->ReadBytes(&stop_waiting->entropy_hash, 1)) {
      set_detailed_error("Unable to read entropy hash for sent packets.");
      return false;
    }
  
    QuicPacketNumber least_unacked_delta = 0;
    if (!reader->ReadBytes(&least_unacked_delta,
                           header.public_header.packet_number_length)) {
      set_detailed_error("Unable to read least unacked delta.");
      return false;
    }
    DCHECK_GE(header.packet_number, least_unacked_delta);
    stop_waiting->least_unacked = header.packet_number - least_unacked_delta;
  
    return true;
  }
  
  bool QuicFramer::ProcessRstStreamFrame(QuicDataReader* reader,
                                         QuicRstStreamFrame* frame) {
    if (!reader->ReadUInt32(&frame->stream_id)) {
      set_detailed_error("Unable to read stream_id.");
      return false;
    }
  
    if (!reader->ReadUInt64(&frame->byte_offset)) {
      set_detailed_error("Unable to read rst stream sent byte offset.");
      return false;
    }
  
    uint32_t error_code;
    if (!reader->ReadUInt32(&error_code)) {
      set_detailed_error("Unable to read rst stream error code.");
      return false;
    }
  
    if (error_code >= QUIC_STREAM_LAST_ERROR) {
      set_detailed_error("Invalid rst stream error code.");
      return false;
    }
  
    frame->error_code = static_cast<QuicRstStreamErrorCode>(error_code);
    return true;
  }
  
  bool QuicFramer::ProcessConnectionCloseFrame(QuicDataReader* reader,
                                               QuicConnectionCloseFrame* frame) {
    uint32_t error_code;
    if (!reader->ReadUInt32(&error_code)) {
      set_detailed_error("Unable to read connection close error code.");
      return false;
    }
  
    if (error_code >= QUIC_LAST_ERROR) {
      set_detailed_error("Invalid error code.");
      return false;
    }
  
    frame->error_code = static_cast<QuicErrorCode>(error_code);
  
    StringPiece error_details;
    if (!reader->ReadStringPiece16(&error_details)) {
      set_detailed_error("Unable to read connection close error details.");
      return false;
    }
    frame->error_details = error_details.as_string();
  
    return true;
  }
  
  bool QuicFramer::ProcessGoAwayFrame(QuicDataReader* reader,
                                      QuicGoAwayFrame* frame) {
    uint32_t error_code;
    if (!reader->ReadUInt32(&error_code)) {
      set_detailed_error("Unable to read go away error code.");
      return false;
    }
    frame->error_code = static_cast<QuicErrorCode>(error_code);
  
    if (error_code >= QUIC_LAST_ERROR) {
      set_detailed_error("Invalid error code.");
      return false;
    }
  
    uint32_t stream_id;
    if (!reader->ReadUInt32(&stream_id)) {
      set_detailed_error("Unable to read last good stream id.");
      return false;
    }
    frame->last_good_stream_id = static_cast<QuicStreamId>(stream_id);
  
    StringPiece reason_phrase;
    if (!reader->ReadStringPiece16(&reason_phrase)) {
      set_detailed_error("Unable to read goaway reason.");
      return false;
    }
    frame->reason_phrase = reason_phrase.as_string();
  
    return true;
  }
  
  bool QuicFramer::ProcessWindowUpdateFrame(QuicDataReader* reader,
                                            QuicWindowUpdateFrame* frame) {
    if (!reader->ReadUInt32(&frame->stream_id)) {
      set_detailed_error("Unable to read stream_id.");
      return false;
    }
  
    if (!reader->ReadUInt64(&frame->byte_offset)) {
      set_detailed_error("Unable to read window byte_offset.");
      return false;
    }
  
    return true;
  }
  
  bool QuicFramer::ProcessBlockedFrame(QuicDataReader* reader,
                                       QuicBlockedFrame* frame) {
    if (!reader->ReadUInt32(&frame->stream_id)) {
      set_detailed_error("Unable to read stream_id.");
      return false;
    }
  
    return true;
  }
  
  // static
  StringPiece QuicFramer::GetAssociatedDataFromEncryptedPacket(
      const QuicEncryptedPacket& encrypted,
      QuicConnectionIdLength connection_id_length,
      bool includes_version,
      QuicPacketNumberLength packet_number_length) {
    // TODO(ianswett): This is identical to QuicData::AssociatedData.
    return StringPiece(
        encrypted.data(),
        GetStartOfEncryptedData(connection_id_length, includes_version,
                                packet_number_length));
  }
  
  void QuicFramer::SetDecrypter(EncryptionLevel level, QuicDecrypter* decrypter) {
    DCHECK(alternative_decrypter_.get() == nullptr);
    DCHECK_GE(level, decrypter_level_);
    decrypter_.reset(decrypter);
    decrypter_level_ = level;
  }
  
  void QuicFramer::SetAlternativeDecrypter(EncryptionLevel level,
                                           QuicDecrypter* decrypter,
                                           bool latch_once_used) {
    alternative_decrypter_.reset(decrypter);
    alternative_decrypter_level_ = level;
    alternative_decrypter_latch_ = latch_once_used;
  }
  
  const QuicDecrypter* QuicFramer::decrypter() const {
    return decrypter_.get();
  }
  
  const QuicDecrypter* QuicFramer::alternative_decrypter() const {
    return alternative_decrypter_.get();
  }
  
  void QuicFramer::SetEncrypter(EncryptionLevel level, QuicEncrypter* encrypter) {
    DCHECK_GE(level, 0);
    DCHECK_LT(level, NUM_ENCRYPTION_LEVELS);
    encrypter_[level].reset(encrypter);
  }
  
  size_t QuicFramer::EncryptPayload(EncryptionLevel level,
                                    QuicPacketNumber packet_number,
                                    const QuicPacket& packet,
                                    char* buffer,
                                    size_t buffer_len) {
    DCHECK(encrypter_[level].get() != nullptr);
  
    StringPiece associated_data = packet.AssociatedData();
    // Copy in the header, because the encrypter only populates the encrypted
    // plaintext content.
    const size_t ad_len = associated_data.length();
    memcpy(buffer, associated_data.data(), ad_len);
    // Encrypt the plaintext into the buffer.
    size_t output_length = 0;
    if (!encrypter_[level]->EncryptPacket(packet_number, associated_data,
                                          packet.Plaintext(), buffer + ad_len,
                                          &output_length, buffer_len - ad_len)) {
      RaiseError(QUIC_ENCRYPTION_FAILURE);
      return 0;
    }
  
    return ad_len + output_length;
  }
  
  size_t QuicFramer::GetMaxPlaintextSize(size_t ciphertext_size) {
    // In order to keep the code simple, we don't have the current encryption
    // level to hand. Both the NullEncrypter and AES-GCM have a tag length of 12.
    size_t min_plaintext_size = ciphertext_size;
  
    for (int i = ENCRYPTION_NONE; i < NUM_ENCRYPTION_LEVELS; i++) {
      if (encrypter_[i].get() != nullptr) {
        size_t size = encrypter_[i]->GetMaxPlaintextSize(ciphertext_size);
        if (size < min_plaintext_size) {
          min_plaintext_size = size;
        }
      }
    }
  
    return min_plaintext_size;
  }
  
  bool QuicFramer::DecryptPayload(QuicDataReader* encrypted_reader,
                                  const QuicPacketHeader& header,
                                  const QuicEncryptedPacket& packet,
                                  char* decrypted_buffer,
                                  size_t buffer_length,
                                  size_t* decrypted_length) {
    StringPiece encrypted = encrypted_reader->ReadRemainingPayload();
    DCHECK(decrypter_.get() != nullptr);
    const StringPiece& associated_data = GetAssociatedDataFromEncryptedPacket(
        packet, header.public_header.connection_id_length,
        header.public_header.version_flag,
        header.public_header.packet_number_length);
    bool success = decrypter_->DecryptPacket(
        header.packet_number, associated_data, encrypted, decrypted_buffer,
        decrypted_length, buffer_length);
    if (success) {
      visitor_->OnDecryptedPacket(decrypter_level_);
    } else if (alternative_decrypter_.get() != nullptr) {
      success = alternative_decrypter_->DecryptPacket(
          header.packet_number, associated_data, encrypted, decrypted_buffer,
          decrypted_length, buffer_length);
      if (success) {
        visitor_->OnDecryptedPacket(alternative_decrypter_level_);
        if (alternative_decrypter_latch_) {
          // Switch to the alternative decrypter and latch so that we cannot
          // switch back.
          decrypter_.reset(alternative_decrypter_.release());
          decrypter_level_ = alternative_decrypter_level_;
          alternative_decrypter_level_ = ENCRYPTION_NONE;
        } else {
          // Switch the alternative decrypter so that we use it first next time.
          decrypter_.swap(alternative_decrypter_);
          EncryptionLevel level = alternative_decrypter_level_;
          alternative_decrypter_level_ = decrypter_level_;
          decrypter_level_ = level;
        }
      }
    }
  
    if (!success) {
      DLOG(WARNING) << "DecryptPacket failed for packet_number:"
                    << header.packet_number;
      return false;
    }
  
    return true;
  }
  
  size_t QuicFramer::GetAckFrameSize(
      const QuicAckFrame& ack,
      QuicPacketNumberLength packet_number_length) {
    AckFrameInfo ack_info = GetAckFrameInfo(ack);
    QuicPacketNumberLength largest_observed_length =
        GetMinSequenceNumberLength(ack.largest_observed);
    QuicPacketNumberLength missing_packet_number_length =
        GetMinSequenceNumberLength(ack_info.max_delta);
  
    size_t ack_size = GetMinAckFrameSize(largest_observed_length);
    if (!ack_info.nack_ranges.empty()) {
      ack_size += kNumberOfNackRangesSize + kNumberOfRevivedPacketsSize;
      ack_size += min(ack_info.nack_ranges.size(), kMaxNackRanges) *
                  (missing_packet_number_length + PACKET_1BYTE_PACKET_NUMBER);
      if (ack.latest_revived_packet != 0) {
        ack_size += largest_observed_length;
      }
    }
  
    // In version 23, if the ack will be truncated due to too many nack ranges,
    // then do not include the number of timestamps (1 byte).
    if (ack_info.nack_ranges.size() <= kMaxNackRanges) {
      // 1 byte for the number of timestamps.
      ack_size += 1;
      if (ack.received_packet_times.size() > 0) {
        // 1 byte for packet number, 4 bytes for timestamp for the first
        // packet.
        ack_size += 5;
  
        // 1 byte for packet number, 2 bytes for timestamp for the other
        // packets.
        ack_size += 3 * (ack.received_packet_times.size() - 1);
      }
    }
  
    return ack_size;
  }
  
  size_t QuicFramer::ComputeFrameLength(
      const QuicFrame& frame,
      bool last_frame_in_packet,
      InFecGroup is_in_fec_group,
      QuicPacketNumberLength packet_number_length) {
    switch (frame.type) {
      case STREAM_FRAME:
        return GetMinStreamFrameSize(frame.stream_frame->stream_id,
                                     frame.stream_frame->offset,
                                     last_frame_in_packet, is_in_fec_group) +
               frame.stream_frame->frame_length;
      case ACK_FRAME: {
        return GetAckFrameSize(*frame.ack_frame, packet_number_length);
      }
      case STOP_WAITING_FRAME:
        return GetStopWaitingFrameSize(packet_number_length);
      case MTU_DISCOVERY_FRAME:
      // MTU discovery frames are serialized as ping frames.
      case PING_FRAME:
        // Ping has no payload.
        return kQuicFrameTypeSize;
      case RST_STREAM_FRAME:
        return GetRstStreamFrameSize();
      case CONNECTION_CLOSE_FRAME:
        return GetMinConnectionCloseFrameSize() +
               frame.connection_close_frame->error_details.size();
      case GOAWAY_FRAME:
        return GetMinGoAwayFrameSize() + frame.goaway_frame->reason_phrase.size();
      case WINDOW_UPDATE_FRAME:
        return GetWindowUpdateFrameSize();
      case BLOCKED_FRAME:
        return GetBlockedFrameSize();
      case PADDING_FRAME:
        DCHECK(false);
        return 0;
      case NUM_FRAME_TYPES:
        DCHECK(false);
        return 0;
    }
  
    // Not reachable, but some Chrome compilers can't figure that out.  *sigh*
    DCHECK(false);
    return 0;
  }
  
  bool QuicFramer::AppendTypeByte(const QuicFrame& frame,
                                  bool no_stream_frame_length,
                                  QuicDataWriter* writer) {
    uint8_t type_byte = 0;
    switch (frame.type) {
      case STREAM_FRAME: {
        if (frame.stream_frame == nullptr) {
          LOG(DFATAL) << "Failed to append STREAM frame with no stream_frame.";
        }
        // Fin bit.
        type_byte |= frame.stream_frame->fin ? kQuicStreamFinMask : 0;
  
        // Data Length bit.
        type_byte <<= kQuicStreamDataLengthShift;
        type_byte |= no_stream_frame_length ? 0 : kQuicStreamDataLengthMask;
  
        // Offset 3 bits.
        type_byte <<= kQuicStreamOffsetShift;
        const size_t offset_len = GetStreamOffsetSize(frame.stream_frame->offset);
        if (offset_len > 0) {
          type_byte |= offset_len - 1;
        }
  
        // stream id 2 bits.
        type_byte <<= kQuicStreamIdShift;
        type_byte |= GetStreamIdSize(frame.stream_frame->stream_id) - 1;
        type_byte |= kQuicFrameTypeStreamMask;  // Set Stream Frame Type to 1.
        break;
      }
      case ACK_FRAME:
        return true;
      case MTU_DISCOVERY_FRAME:
        type_byte = static_cast<uint8_t>(PING_FRAME);
        break;
      default:
        type_byte = static_cast<uint8_t>(frame.type);
        break;
    }
  
    return writer->WriteUInt8(type_byte);
  }
  
  // static
  bool QuicFramer::AppendPacketSequenceNumber(
      QuicPacketNumberLength packet_number_length,
      QuicPacketNumber packet_number,
      QuicDataWriter* writer) {
    // Ensure the entire packet number can be written.
    if (writer->capacity() - writer->length() <
        static_cast<size_t>(packet_number_length)) {
      return false;
    }
    switch (packet_number_length) {
      case PACKET_1BYTE_PACKET_NUMBER:
        return writer->WriteUInt8(packet_number & k1ByteSequenceNumberMask);
        break;
      case PACKET_2BYTE_PACKET_NUMBER:
        return writer->WriteUInt16(packet_number & k2ByteSequenceNumberMask);
        break;
      case PACKET_4BYTE_PACKET_NUMBER:
        return writer->WriteUInt32(packet_number & k4ByteSequenceNumberMask);
        break;
      case PACKET_6BYTE_PACKET_NUMBER:
        return writer->WriteUInt48(packet_number & k6ByteSequenceNumberMask);
        break;
      default:
        DCHECK(false) << "packet_number_length: " << packet_number_length;
        return false;
    }
  }
  
  bool QuicFramer::AppendStreamFrame(const QuicStreamFrame& frame,
                                     bool no_stream_frame_length,
                                     QuicDataWriter* writer) {
    if (!writer->WriteBytes(&frame.stream_id, GetStreamIdSize(frame.stream_id))) {
      LOG(DFATAL) << "Writing stream id size failed.";
      return false;
    }
    if (!writer->WriteBytes(&frame.offset, GetStreamOffsetSize(frame.offset))) {
      LOG(DFATAL) << "Writing offset size failed.";
      return false;
    }
    if (!no_stream_frame_length) {
      if ((frame.frame_length > numeric_limits<uint16_t>::max()) ||
          !writer->WriteUInt16(static_cast<uint16_t>(frame.frame_length))) {
        LOG(DFATAL) << "Writing stream frame length failed";
        return false;
      }
    }
  
    if (!writer->WriteBytes(frame.frame_buffer, frame.frame_length)) {
      LOG(DFATAL) << "Writing frame data failed.";
      return false;
    }
    return true;
  }
  
  void QuicFramer::set_version(const QuicVersion version) {
    DCHECK(IsSupportedVersion(version)) << QuicVersionToString(version);
    quic_version_ = version;
  }
  
  bool QuicFramer::AppendAckFrameAndTypeByte(const QuicPacketHeader& header,
                                             const QuicAckFrame& frame,
                                             QuicDataWriter* writer) {
    AckFrameInfo ack_info = GetAckFrameInfo(frame);
    QuicPacketNumber ack_largest_observed = frame.largest_observed;
    QuicPacketNumberLength largest_observed_length =
        GetMinSequenceNumberLength(ack_largest_observed);
    QuicPacketNumberLength missing_packet_number_length =
        GetMinSequenceNumberLength(ack_info.max_delta);
    // Determine whether we need to truncate ranges.
    size_t available_range_bytes =
        writer->capacity() - writer->length() - kNumberOfRevivedPacketsSize -
        kNumberOfNackRangesSize - GetMinAckFrameSize(largest_observed_length);
    size_t max_num_ranges =
        available_range_bytes /
        (missing_packet_number_length + PACKET_1BYTE_PACKET_NUMBER);
    max_num_ranges = min(kMaxNackRanges, max_num_ranges);
    bool truncated = ack_info.nack_ranges.size() > max_num_ranges;
    DVLOG_IF(1, truncated) << "Truncating ack from "
                           << ack_info.nack_ranges.size() << " ranges to "
                           << max_num_ranges;
    // Write out the type byte by setting the low order bits and doing shifts
    // to make room for the next bit flags to be set.
    // Whether there are any nacks.
    uint8_t type_byte = ack_info.nack_ranges.empty() ? 0 : kQuicHasNacksMask;
  
    // truncating bit.
    type_byte <<= kQuicAckTruncatedShift;
    type_byte |= truncated ? kQuicAckTruncatedMask : 0;
  
    // Largest observed packet number length.
    type_byte <<= kQuicSequenceNumberLengthShift;
    type_byte |= GetSequenceNumberFlags(largest_observed_length);
  
    // Missing packet number length.
    type_byte <<= kQuicSequenceNumberLengthShift;
    type_byte |= GetSequenceNumberFlags(missing_packet_number_length);
  
    type_byte |= kQuicFrameTypeAckMask;
  
    if (!writer->WriteUInt8(type_byte)) {
      return false;
    }
  
    QuicPacketEntropyHash ack_entropy_hash = frame.entropy_hash;
    NackRangeMap::reverse_iterator ack_iter = ack_info.nack_ranges.rbegin();
    if (truncated) {
      // Skip the nack ranges which the truncated ack won't include and set
      // a correct largest observed for the truncated ack.
      for (size_t i = 1; i < (ack_info.nack_ranges.size() - max_num_ranges);
           ++i) {
        ++ack_iter;
      }
      // If the last range is followed by acks, include them.
      // If the last range is followed by another range, specify the end of the
      // range as the largest_observed.
      ack_largest_observed = ack_iter->first - 1;
      // Also update the entropy so it matches the largest observed.
      ack_entropy_hash = entropy_calculator_->EntropyHash(ack_largest_observed);
      ++ack_iter;
    }
  
    if (!writer->WriteUInt8(ack_entropy_hash)) {
      return false;
    }
  
    if (!AppendPacketSequenceNumber(largest_observed_length, ack_largest_observed,
                                    writer)) {
      return false;
    }
  
    uint64_t delta_time_largest_observed_us = kUFloat16MaxValue;
    if (!frame.delta_time_largest_observed.IsInfinite()) {
      DCHECK_LE(0u, frame.delta_time_largest_observed.ToMicroseconds());
      delta_time_largest_observed_us =
          frame.delta_time_largest_observed.ToMicroseconds();
    }
  
    if (!writer->WriteUFloat16(delta_time_largest_observed_us)) {
      return false;
    }
  
    // Timestamp goes at the end of the required fields.
    if (!truncated) {
      if (!AppendTimestampToAckFrame(frame, writer)) {
        return false;
      }
    }
  
    if (ack_info.nack_ranges.empty()) {
      return true;
    }
  
    const uint8_t num_missing_ranges =
        static_cast<uint8_t>(min(ack_info.nack_ranges.size(), max_num_ranges));
    if (!writer->WriteBytes(&num_missing_ranges, 1)) {
      return false;
    }
  
    int num_ranges_written = 0;
    QuicPacketNumber last_sequence_written = ack_largest_observed;
    for (; ack_iter != ack_info.nack_ranges.rend(); ++ack_iter) {
      // Calculate the delta to the last number in the range.
      QuicPacketNumber missing_delta =
          last_sequence_written - (ack_iter->first + ack_iter->second);
      if (!AppendPacketSequenceNumber(missing_packet_number_length, missing_delta,
                                      writer)) {
        return false;
      }
      if (!AppendPacketSequenceNumber(PACKET_1BYTE_PACKET_NUMBER,
                                      ack_iter->second, writer)) {
        return false;
      }
      // Subtract 1 so a missing_delta of 0 means an adjacent range.
      last_sequence_written = ack_iter->first - 1;
      ++num_ranges_written;
    }
    DCHECK_EQ(num_missing_ranges, num_ranges_written);
  
    // Append revived packets.
    // If not all the revived packets fit, only mention the ones that do.
    uint8_t num_revived_packets = frame.latest_revived_packet == 0 ? 0 : 1;
    if (((writer->capacity() - writer->length()) / largest_observed_length) ==
        0) {
      num_revived_packets = 0;
    }
    if (!writer->WriteBytes(&num_revived_packets, 1)) {
      return false;
    }
  
    if (num_revived_packets > 0) {
      LOG_IF(DFATAL,
             !frame.missing_packets.Contains(frame.latest_revived_packet));
      if (!AppendPacketSequenceNumber(largest_observed_length,
                                      frame.latest_revived_packet, writer)) {
        return false;
      }
    }
  
    return true;
  }
  
  bool QuicFramer::AppendTimestampToAckFrame(const QuicAckFrame& frame,
                                             QuicDataWriter* writer) {
    DCHECK_GE(numeric_limits<uint8_t>::max(), frame.received_packet_times.size());
    // num_received_packets is only 1 byte.
    if (frame.received_packet_times.size() > numeric_limits<uint8_t>::max()) {
      return false;
    }
  
    uint8_t num_received_packets = frame.received_packet_times.size();
    if (!writer->WriteBytes(&num_received_packets, 1)) {
      return false;
    }
    if (num_received_packets == 0) {
      return true;
    }
  
    PacketTimeVector::const_iterator it = frame.received_packet_times.begin();
    QuicPacketNumber packet_number = it->first;
    QuicPacketNumber delta_from_largest_observed =
        frame.largest_observed - packet_number;
  
    DCHECK_GE(numeric_limits<uint8_t>::max(), delta_from_largest_observed);
    if (delta_from_largest_observed > numeric_limits<uint8_t>::max()) {
      return false;
    }
  
    if (!writer->WriteUInt8(delta_from_largest_observed &
                            k1ByteSequenceNumberMask)) {
      return false;
    }
  
    // Use the lowest 4 bytes of the time delta from the creation_time_.
    const uint64_t time_epoch_delta_us = UINT64_C(1) << 32;
    uint32_t time_delta_us = static_cast<uint32_t>(
        it->second.Subtract(creation_time_).ToMicroseconds() &
        (time_epoch_delta_us - 1));
    if (!writer->WriteBytes(&time_delta_us, sizeof(time_delta_us))) {
      return false;
    }
  
    QuicTime prev_time = it->second;
  
    for (++it; it != frame.received_packet_times.end(); ++it) {
      packet_number = it->first;
      delta_from_largest_observed = frame.largest_observed - packet_number;
  
      if (delta_from_largest_observed > numeric_limits<uint8_t>::max()) {
        return false;
      }
  
      if (!writer->WriteUInt8(delta_from_largest_observed &
                              k1ByteSequenceNumberMask)) {
        return false;
      }
  
      uint64_t frame_time_delta_us =
          it->second.Subtract(prev_time).ToMicroseconds();
      prev_time = it->second;
      if (!writer->WriteUFloat16(frame_time_delta_us)) {
        return false;
      }
    }
    return true;
  }
  
  bool QuicFramer::AppendStopWaitingFrame(const QuicPacketHeader& header,
                                          const QuicStopWaitingFrame& frame,
                                          QuicDataWriter* writer) {
    DCHECK_GE(header.packet_number, frame.least_unacked);
    const QuicPacketNumber least_unacked_delta =
        header.packet_number - frame.least_unacked;
    const QuicPacketNumber length_shift =
        header.public_header.packet_number_length * 8;
    if (!writer->WriteUInt8(frame.entropy_hash)) {
      LOG(DFATAL) << " hash failed";
      return false;
    }
  
    if (least_unacked_delta >> length_shift > 0) {
      LOG(DFATAL) << "packet_number_length "
                  << header.public_header.packet_number_length
                  << " is too small for least_unacked_delta: "
                  << least_unacked_delta;
      return false;
    }
    if (!AppendPacketSequenceNumber(header.public_header.packet_number_length,
                                    least_unacked_delta, writer)) {
      LOG(DFATAL) << " seq failed: " << header.public_header.packet_number_length;
      return false;
    }
  
    return true;
  }
  
  bool QuicFramer::AppendRstStreamFrame(const QuicRstStreamFrame& frame,
                                        QuicDataWriter* writer) {
    if (!writer->WriteUInt32(frame.stream_id)) {
      return false;
    }
  
    if (!writer->WriteUInt64(frame.byte_offset)) {
      return false;
    }
  
    uint32_t error_code = static_cast<uint32_t>(frame.error_code);
    if (!writer->WriteUInt32(error_code)) {
      return false;
    }
  
    return true;
  }
  
  bool QuicFramer::AppendConnectionCloseFrame(
      const QuicConnectionCloseFrame& frame,
      QuicDataWriter* writer) {
    uint32_t error_code = static_cast<uint32_t>(frame.error_code);
    if (!writer->WriteUInt32(error_code)) {
      return false;
    }
    if (!writer->WriteStringPiece16(frame.error_details)) {
      return false;
    }
    return true;
  }
  
  bool QuicFramer::AppendGoAwayFrame(const QuicGoAwayFrame& frame,
                                     QuicDataWriter* writer) {
    uint32_t error_code = static_cast<uint32_t>(frame.error_code);
    if (!writer->WriteUInt32(error_code)) {
      return false;
    }
    uint32_t stream_id = static_cast<uint32_t>(frame.last_good_stream_id);
    if (!writer->WriteUInt32(stream_id)) {
      return false;
    }
    if (!writer->WriteStringPiece16(frame.reason_phrase)) {
      return false;
    }
    return true;
  }
  
  bool QuicFramer::AppendWindowUpdateFrame(const QuicWindowUpdateFrame& frame,
                                           QuicDataWriter* writer) {
    uint32_t stream_id = static_cast<uint32_t>(frame.stream_id);
    if (!writer->WriteUInt32(stream_id)) {
      return false;
    }
    if (!writer->WriteUInt64(frame.byte_offset)) {
      return false;
    }
    return true;
  }
  
  bool QuicFramer::AppendBlockedFrame(const QuicBlockedFrame& frame,
                                      QuicDataWriter* writer) {
    uint32_t stream_id = static_cast<uint32_t>(frame.stream_id);
    if (!writer->WriteUInt32(stream_id)) {
      return false;
    }
    return true;
  }
  
  bool QuicFramer::RaiseError(QuicErrorCode error) {
    DVLOG(1) << "Error: " << QuicUtils::ErrorToString(error)
             << " detail: " << detailed_error_;
    set_error(error);
    visitor_->OnError(this);
    return false;
  }
  
  }  // namespace net
  