modules/rtp_rtcp/source/flexfec_header_reader_writer.cc - src.git - Git at Google

 /*
  *  Copyright (c) 2016 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 "modules/rtp_rtcp/source/flexfec_header_reader_writer.h"

 #include <string.h>

 #include "api/scoped_refptr.h"
 #include "modules/rtp_rtcp/source/byte_io.h"
 #include "modules/rtp_rtcp/source/forward_error_correction_internal.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"

 namespace webrtc {

 namespace {

 // Maximum number of media packets that can be protected in one batch.
 constexpr size_t kMaxMediaPackets = 48;  // Since we are reusing ULPFEC masks.

 // Maximum number of FEC packets stored inside ForwardErrorCorrection.
 constexpr size_t kMaxFecPackets = kMaxMediaPackets;

 // Size (in bytes) of packet masks, given number of K bits set.
 constexpr size_t kFlexfecPacketMaskSizes[] = {2, 6, 14};

 // Size (in bytes) of part of header which is not packet mask specific.
 constexpr size_t kBaseHeaderSize = 12;

 // Size (in bytes) of part of header which is stream specific.
 constexpr size_t kStreamSpecificHeaderSize = 6;

 // Size (in bytes) of header, given the single stream packet mask size, i.e.
 // the number of K-bits set.
 constexpr size_t kHeaderSizes[] = {
     kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[0],
     kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[1],
     kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[2]};

 // We currently only support single-stream protection.
 // TODO(brandtr): Update this when we support multistream protection.
 constexpr uint8_t kSsrcCount = 1;

 // There are three reserved bytes that MUST be set to zero in the header.
 constexpr uint32_t kReservedBits = 0;

 // TODO(brandtr): Update this when we support multistream protection.
 constexpr size_t kPacketMaskOffset =
     kBaseHeaderSize + kStreamSpecificHeaderSize;

 // Here we count the K-bits as belonging to the packet mask.
 // This can be used in conjunction with FlexfecHeaderWriter::MinPacketMaskSize,
 // which calculates a bound on the needed packet mask size including K-bits,
 // given a packet mask without K-bits.
 size_t FlexfecHeaderSize(size_t packet_mask_size) {
   RTC_DCHECK_LE(packet_mask_size, kFlexfecPacketMaskSizes[2]);
   if (packet_mask_size <= kFlexfecPacketMaskSizes[0]) {
     return kHeaderSizes[0];
   } else if (packet_mask_size <= kFlexfecPacketMaskSizes[1]) {
     return kHeaderSizes[1];
   }
   return kHeaderSizes[2];
 }

 }  // namespace

 FlexfecHeaderReader::FlexfecHeaderReader()
     : FecHeaderReader(kMaxMediaPackets, kMaxFecPackets) {}

 FlexfecHeaderReader::~FlexfecHeaderReader() = default;

 // TODO(brandtr): Update this function when we support flexible masks,
 // retransmissions, and/or several protected SSRCs.
 bool FlexfecHeaderReader::ReadFecHeader(
     ForwardErrorCorrection::ReceivedFecPacket* fec_packet) const {
   if (fec_packet->pkt->length <= kBaseHeaderSize + kStreamSpecificHeaderSize) {
     RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
     return false;
   }
   bool r_bit = (fec_packet->pkt->data[0] & 0x80) != 0;
   if (r_bit) {
     RTC_LOG(LS_INFO)
         << "FlexFEC packet with retransmission bit set. We do not yet "
            "support this, thus discarding the packet.";
     return false;
   }
   bool f_bit = (fec_packet->pkt->data[0] & 0x40) != 0;
   if (f_bit) {
     RTC_LOG(LS_INFO)
         << "FlexFEC packet with inflexible generator matrix. We do "
            "not yet support this, thus discarding packet.";
     return false;
   }
   uint8_t ssrc_count =
       ByteReader<uint8_t>::ReadBigEndian(&fec_packet->pkt->data[8]);
   if (ssrc_count != 1) {
     RTC_LOG(LS_INFO)
         << "FlexFEC packet protecting multiple media SSRCs. We do not "
            "yet support this, thus discarding packet.";
     return false;
   }
   uint32_t protected_ssrc =
       ByteReader<uint32_t>::ReadBigEndian(&fec_packet->pkt->data[12]);
   uint16_t seq_num_base =
       ByteReader<uint16_t>::ReadBigEndian(&fec_packet->pkt->data[16]);

   // Parse the FlexFEC packet mask and remove the interleaved K-bits.
   // (See FEC header schematic in flexfec_header_reader_writer.h.)
   // We store the packed packet mask in-band, which "destroys" the standards
   // compliance of the header. That is fine though, since the code that
   // reads from the header (from this point and onwards) is aware of this.
   // TODO(brandtr): When the FEC packet classes have been refactored, store
   // the packed packet masks out-of-band, thus leaving the FlexFEC header as is.
   //
   // We treat the mask parts as unsigned integers with host order endianness
   // in order to simplify the bit shifting between bytes.
   if (fec_packet->pkt->length < kHeaderSizes[0]) {
     RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
     return false;
   }
   uint8_t* const packet_mask = fec_packet->pkt->data + kPacketMaskOffset;
   bool k_bit0 = (packet_mask[0] & 0x80) != 0;
   uint16_t mask_part0 = ByteReader<uint16_t>::ReadBigEndian(&packet_mask[0]);
   // Shift away K-bit 0, implicitly clearing the last bit.
   mask_part0 <<= 1;
   ByteWriter<uint16_t>::WriteBigEndian(&packet_mask[0], mask_part0);
   size_t packet_mask_size;
   if (k_bit0) {
     // The first K-bit is set, and the packet mask is thus only 2 bytes long.
     // We have now read the entire FEC header, and the rest of the packet
     // is payload.
     packet_mask_size = kFlexfecPacketMaskSizes[0];
   } else {
     if (fec_packet->pkt->length < kHeaderSizes[1]) {
       return false;
     }
     bool k_bit1 = (packet_mask[2] & 0x80) != 0;
     // We have already shifted the first two bytes of the packet mask one step
     // to the left, thus removing K-bit 0. We will now shift the next four bytes
     // of the packet mask two steps to the left. (One step for the removed
     // K-bit 0, and one step for the to be removed K-bit 1).
     uint8_t bit15 = (packet_mask[2] >> 6) & 0x01;
     packet_mask[1] |= bit15;
     uint32_t mask_part1 = ByteReader<uint32_t>::ReadBigEndian(&packet_mask[2]);
     // Shift away K-bit 1 and bit 15, implicitly clearing the last two bits.
     mask_part1 <<= 2;
     ByteWriter<uint32_t>::WriteBigEndian(&packet_mask[2], mask_part1);
     if (k_bit1) {
       // The first K-bit is clear, but the second K-bit is set. The packet
       // mask is thus 6 bytes long.  We have now read the entire FEC header,
       // and the rest of the packet is payload.
       packet_mask_size = kFlexfecPacketMaskSizes[1];
     } else {
       if (fec_packet->pkt->length < kHeaderSizes[2]) {
         RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
         return false;
       }
       bool k_bit2 = (packet_mask[6] & 0x80) != 0;
       if (k_bit2) {
         // The first and second K-bits are clear, but the third K-bit is set.
         // The packet mask is thus 14 bytes long. We have now read the entire
         // FEC header, and the rest of the packet is payload.
         packet_mask_size = kFlexfecPacketMaskSizes[2];
       } else {
         RTC_LOG(LS_WARNING)
             << "Discarding FlexFEC packet with malformed header.";
         return false;
       }
       // At this point, K-bits 0 and 1 have been removed, and the front-most
       // part of the FlexFEC packet mask has been packed accordingly. We will
       // now shift the remaning part of the packet mask three steps to the left.
       // This corresponds to the (in total) three K-bits, which have been
       // removed.
       uint8_t tail_bits = (packet_mask[6] >> 5) & 0x03;
       packet_mask[5] |= tail_bits;
       uint64_t mask_part2 =
           ByteReader<uint64_t>::ReadBigEndian(&packet_mask[6]);
       // Shift away K-bit 2, bit 46, and bit 47, implicitly clearing the last
       // three bits.
       mask_part2 <<= 3;
       ByteWriter<uint64_t>::WriteBigEndian(&packet_mask[6], mask_part2);
     }
   }

   // Store "ULPFECized" packet mask info.
   fec_packet->fec_header_size = FlexfecHeaderSize(packet_mask_size);
   fec_packet->protected_ssrc = protected_ssrc;
   fec_packet->seq_num_base = seq_num_base;
   fec_packet->packet_mask_offset = kPacketMaskOffset;
   fec_packet->packet_mask_size = packet_mask_size;

   // In FlexFEC, all media packets are protected in their entirety.
   fec_packet->protection_length =
       fec_packet->pkt->length - fec_packet->fec_header_size;

   return true;
 }

 FlexfecHeaderWriter::FlexfecHeaderWriter()
     : FecHeaderWriter(kMaxMediaPackets, kMaxFecPackets, kHeaderSizes[2]) {}

 FlexfecHeaderWriter::~FlexfecHeaderWriter() = default;

 size_t FlexfecHeaderWriter::MinPacketMaskSize(const uint8_t* packet_mask,
                                               size_t packet_mask_size) const {
   if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear &&
       (packet_mask[1] & 0x01) == 0) {
     // Packet mask is 16 bits long, with bit 15 clear.
     // It can be used as is.
     return kFlexfecPacketMaskSizes[0];
   } else if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear) {
     // Packet mask is 16 bits long, with bit 15 set.
     // We must expand the packet mask with zeros in the FlexFEC header.
     return kFlexfecPacketMaskSizes[1];
   } else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet &&
              (packet_mask[5] & 0x03) == 0) {
     // Packet mask is 48 bits long, with bits 46 and 47 clear.
     // It can be used as is.
     return kFlexfecPacketMaskSizes[1];
   } else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet) {
     // Packet mask is 48 bits long, with at least one of bits 46 and 47 set.
     // We must expand it with zeros.
     return kFlexfecPacketMaskSizes[2];
   }
   RTC_NOTREACHED() << "Incorrect packet mask size: " << packet_mask_size << ".";
   return kFlexfecPacketMaskSizes[2];
 }

 size_t FlexfecHeaderWriter::FecHeaderSize(size_t packet_mask_size) const {
   return FlexfecHeaderSize(packet_mask_size);
 }

 // This function adapts the precomputed ULPFEC packet masks to the
 // FlexFEC header standard. Note that the header size is computed by
 // FecHeaderSize(), so in this function we can be sure that we are
 // writing in space that is intended for the header.
 //
 // TODO(brandtr): Update this function when we support offset-based masks,
 // retransmissions, and protecting multiple SSRCs.
 void FlexfecHeaderWriter::FinalizeFecHeader(
     uint32_t media_ssrc,
     uint16_t seq_num_base,
     const uint8_t* packet_mask,
     size_t packet_mask_size,
     ForwardErrorCorrection::Packet* fec_packet) const {
   fec_packet->data[0] &= 0x7f;  // Clear R bit.
   fec_packet->data[0] &= 0xbf;  // Clear F bit.
   ByteWriter<uint8_t>::WriteBigEndian(&fec_packet->data[8], kSsrcCount);
   ByteWriter<uint32_t, 3>::WriteBigEndian(&fec_packet->data[9], kReservedBits);
   ByteWriter<uint32_t>::WriteBigEndian(&fec_packet->data[12], media_ssrc);
   ByteWriter<uint16_t>::WriteBigEndian(&fec_packet->data[16], seq_num_base);
   // Adapt ULPFEC packet mask to FlexFEC header.
   //
   // We treat the mask parts as unsigned integers with host order endianness
   // in order to simplify the bit shifting between bytes.
   uint8_t* const written_packet_mask = fec_packet->data + kPacketMaskOffset;
   if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet) {
     // The packet mask is 48 bits long.
     uint16_t tmp_mask_part0 =
         ByteReader<uint16_t>::ReadBigEndian(&packet_mask[0]);
     uint32_t tmp_mask_part1 =
         ByteReader<uint32_t>::ReadBigEndian(&packet_mask[2]);

     tmp_mask_part0 >>= 1;  // Shift, thus clearing K-bit 0.
     ByteWriter<uint16_t>::WriteBigEndian(&written_packet_mask[0],
                                          tmp_mask_part0);
     tmp_mask_part1 >>= 2;  // Shift, thus clearing K-bit 1 and bit 15.
     ByteWriter<uint32_t>::WriteBigEndian(&written_packet_mask[2],
                                          tmp_mask_part1);
     bool bit15 = (packet_mask[1] & 0x01) != 0;
     if (bit15)
       written_packet_mask[2] |= 0x40;  // Set bit 15.
     bool bit46 = (packet_mask[5] & 0x02) != 0;
     bool bit47 = (packet_mask[5] & 0x01) != 0;
     if (!bit46 && !bit47) {
       written_packet_mask[2] |= 0x80;  // Set K-bit 1.
     } else {
       memset(&written_packet_mask[6], 0, 8);  // Clear all trailing bits.
       written_packet_mask[6] |= 0x80;         // Set K-bit 2.
       if (bit46)
         written_packet_mask[6] |= 0x40;  // Set bit 46.
       if (bit47)
         written_packet_mask[6] |= 0x20;  // Set bit 47.
     }
   } else if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear) {
     // The packet mask is 16 bits long.
     uint16_t tmp_mask_part0 =
         ByteReader<uint16_t>::ReadBigEndian(&packet_mask[0]);

     tmp_mask_part0 >>= 1;  // Shift, thus clearing K-bit 0.
     ByteWriter<uint16_t>::WriteBigEndian(&written_packet_mask[0],
                                          tmp_mask_part0);
     bool bit15 = (packet_mask[1] & 0x01) != 0;
     if (!bit15) {
       written_packet_mask[0] |= 0x80;  // Set K-bit 0.
     } else {
       memset(&written_packet_mask[2], 0U, 4);  // Clear all trailing bits.
       written_packet_mask[2] |= 0x80;          // Set K-bit 1.
       written_packet_mask[2] |= 0x40;          // Set bit 15.
     }
   } else {
     RTC_NOTREACHED() << "Incorrect packet mask size: " << packet_mask_size
                      << ".";
   }
 }

 }  // namespace webrtc
	/*
	* Copyright (c) 2016 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 "modules/rtp_rtcp/source/flexfec_header_reader_writer.h"

	#include <string.h>

	#include "api/scoped_refptr.h"
	#include "modules/rtp_rtcp/source/byte_io.h"
	#include "modules/rtp_rtcp/source/forward_error_correction_internal.h"
	#include "rtc_base/checks.h"
	#include "rtc_base/logging.h"

	namespace webrtc {

	namespace {

	// Maximum number of media packets that can be protected in one batch.
	constexpr size_t kMaxMediaPackets = 48; // Since we are reusing ULPFEC masks.

	// Maximum number of FEC packets stored inside ForwardErrorCorrection.
	constexpr size_t kMaxFecPackets = kMaxMediaPackets;

	// Size (in bytes) of packet masks, given number of K bits set.
	constexpr size_t kFlexfecPacketMaskSizes[] = {2, 6, 14};

	// Size (in bytes) of part of header which is not packet mask specific.
	constexpr size_t kBaseHeaderSize = 12;

	// Size (in bytes) of part of header which is stream specific.
	constexpr size_t kStreamSpecificHeaderSize = 6;

	// Size (in bytes) of header, given the single stream packet mask size, i.e.
	// the number of K-bits set.
	constexpr size_t kHeaderSizes[] = {
	kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[0],
	kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[1],
	kBaseHeaderSize + kStreamSpecificHeaderSize + kFlexfecPacketMaskSizes[2]};

	// We currently only support single-stream protection.
	// TODO(brandtr): Update this when we support multistream protection.
	constexpr uint8_t kSsrcCount = 1;

	// There are three reserved bytes that MUST be set to zero in the header.
	constexpr uint32_t kReservedBits = 0;

	// TODO(brandtr): Update this when we support multistream protection.
	constexpr size_t kPacketMaskOffset =
	kBaseHeaderSize + kStreamSpecificHeaderSize;

	// Here we count the K-bits as belonging to the packet mask.
	// This can be used in conjunction with FlexfecHeaderWriter::MinPacketMaskSize,
	// which calculates a bound on the needed packet mask size including K-bits,
	// given a packet mask without K-bits.
	size_t FlexfecHeaderSize(size_t packet_mask_size) {
	RTC_DCHECK_LE(packet_mask_size, kFlexfecPacketMaskSizes[2]);
	if (packet_mask_size <= kFlexfecPacketMaskSizes[0]) {
	return kHeaderSizes[0];
	} else if (packet_mask_size <= kFlexfecPacketMaskSizes[1]) {
	return kHeaderSizes[1];
	}
	return kHeaderSizes[2];
	}

	} // namespace

	FlexfecHeaderReader::FlexfecHeaderReader()
	: FecHeaderReader(kMaxMediaPackets, kMaxFecPackets) {}

	FlexfecHeaderReader::~FlexfecHeaderReader() = default;

	// TODO(brandtr): Update this function when we support flexible masks,
	// retransmissions, and/or several protected SSRCs.
	bool FlexfecHeaderReader::ReadFecHeader(
	ForwardErrorCorrection::ReceivedFecPacket* fec_packet) const {
	if (fec_packet->pkt->length <= kBaseHeaderSize + kStreamSpecificHeaderSize) {
	RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
	return false;
	}
	bool r_bit = (fec_packet->pkt->data[0] & 0x80) != 0;
	if (r_bit) {
	RTC_LOG(LS_INFO)
	<< "FlexFEC packet with retransmission bit set. We do not yet "
	"support this, thus discarding the packet.";
	return false;
	}
	bool f_bit = (fec_packet->pkt->data[0] & 0x40) != 0;
	if (f_bit) {
	RTC_LOG(LS_INFO)
	<< "FlexFEC packet with inflexible generator matrix. We do "
	"not yet support this, thus discarding packet.";
	return false;
	}
	uint8_t ssrc_count =
	ByteReader<uint8_t>::ReadBigEndian(&fec_packet->pkt->data[8]);
	if (ssrc_count != 1) {
	RTC_LOG(LS_INFO)
	<< "FlexFEC packet protecting multiple media SSRCs. We do not "
	"yet support this, thus discarding packet.";
	return false;
	}
	uint32_t protected_ssrc =
	ByteReader<uint32_t>::ReadBigEndian(&fec_packet->pkt->data[12]);
	uint16_t seq_num_base =
	ByteReader<uint16_t>::ReadBigEndian(&fec_packet->pkt->data[16]);

	// Parse the FlexFEC packet mask and remove the interleaved K-bits.
	// (See FEC header schematic in flexfec_header_reader_writer.h.)
	// We store the packed packet mask in-band, which "destroys" the standards
	// compliance of the header. That is fine though, since the code that
	// reads from the header (from this point and onwards) is aware of this.
	// TODO(brandtr): When the FEC packet classes have been refactored, store
	// the packed packet masks out-of-band, thus leaving the FlexFEC header as is.
	//
	// We treat the mask parts as unsigned integers with host order endianness
	// in order to simplify the bit shifting between bytes.
	if (fec_packet->pkt->length < kHeaderSizes[0]) {
	RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
	return false;
	}
	uint8_t* const packet_mask = fec_packet->pkt->data + kPacketMaskOffset;
	bool k_bit0 = (packet_mask[0] & 0x80) != 0;
	uint16_t mask_part0 = ByteReader<uint16_t>::ReadBigEndian(&packet_mask[0]);
	// Shift away K-bit 0, implicitly clearing the last bit.
	mask_part0 <<= 1;
	ByteWriter<uint16_t>::WriteBigEndian(&packet_mask[0], mask_part0);
	size_t packet_mask_size;
	if (k_bit0) {
	// The first K-bit is set, and the packet mask is thus only 2 bytes long.
	// We have now read the entire FEC header, and the rest of the packet
	// is payload.
	packet_mask_size = kFlexfecPacketMaskSizes[0];
	} else {
	if (fec_packet->pkt->length < kHeaderSizes[1]) {
	return false;
	}
	bool k_bit1 = (packet_mask[2] & 0x80) != 0;
	// We have already shifted the first two bytes of the packet mask one step
	// to the left, thus removing K-bit 0. We will now shift the next four bytes
	// of the packet mask two steps to the left. (One step for the removed
	// K-bit 0, and one step for the to be removed K-bit 1).
	uint8_t bit15 = (packet_mask[2] >> 6) & 0x01;
	packet_mask[1] \|= bit15;
	uint32_t mask_part1 = ByteReader<uint32_t>::ReadBigEndian(&packet_mask[2]);
	// Shift away K-bit 1 and bit 15, implicitly clearing the last two bits.
	mask_part1 <<= 2;
	ByteWriter<uint32_t>::WriteBigEndian(&packet_mask[2], mask_part1);
	if (k_bit1) {
	// The first K-bit is clear, but the second K-bit is set. The packet
	// mask is thus 6 bytes long. We have now read the entire FEC header,
	// and the rest of the packet is payload.
	packet_mask_size = kFlexfecPacketMaskSizes[1];
	} else {
	if (fec_packet->pkt->length < kHeaderSizes[2]) {
	RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
	return false;
	}
	bool k_bit2 = (packet_mask[6] & 0x80) != 0;
	if (k_bit2) {
	// The first and second K-bits are clear, but the third K-bit is set.
	// The packet mask is thus 14 bytes long. We have now read the entire
	// FEC header, and the rest of the packet is payload.
	packet_mask_size = kFlexfecPacketMaskSizes[2];
	} else {
	RTC_LOG(LS_WARNING)
	<< "Discarding FlexFEC packet with malformed header.";
	return false;
	}
	// At this point, K-bits 0 and 1 have been removed, and the front-most
	// part of the FlexFEC packet mask has been packed accordingly. We will
	// now shift the remaning part of the packet mask three steps to the left.
	// This corresponds to the (in total) three K-bits, which have been
	// removed.
	uint8_t tail_bits = (packet_mask[6] >> 5) & 0x03;
	packet_mask[5] \|= tail_bits;
	uint64_t mask_part2 =
	ByteReader<uint64_t>::ReadBigEndian(&packet_mask[6]);
	// Shift away K-bit 2, bit 46, and bit 47, implicitly clearing the last
	// three bits.
	mask_part2 <<= 3;
	ByteWriter<uint64_t>::WriteBigEndian(&packet_mask[6], mask_part2);
	}
	}

	// Store "ULPFECized" packet mask info.
	fec_packet->fec_header_size = FlexfecHeaderSize(packet_mask_size);
	fec_packet->protected_ssrc = protected_ssrc;
	fec_packet->seq_num_base = seq_num_base;
	fec_packet->packet_mask_offset = kPacketMaskOffset;
	fec_packet->packet_mask_size = packet_mask_size;

	// In FlexFEC, all media packets are protected in their entirety.
	fec_packet->protection_length =
	fec_packet->pkt->length - fec_packet->fec_header_size;

	return true;
	}

	FlexfecHeaderWriter::FlexfecHeaderWriter()
	: FecHeaderWriter(kMaxMediaPackets, kMaxFecPackets, kHeaderSizes[2]) {}

	FlexfecHeaderWriter::~FlexfecHeaderWriter() = default;

	size_t FlexfecHeaderWriter::MinPacketMaskSize(const uint8_t* packet_mask,
	size_t packet_mask_size) const {
	if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear &&
	(packet_mask[1] & 0x01) == 0) {
	// Packet mask is 16 bits long, with bit 15 clear.
	// It can be used as is.
	return kFlexfecPacketMaskSizes[0];
	} else if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear) {
	// Packet mask is 16 bits long, with bit 15 set.
	// We must expand the packet mask with zeros in the FlexFEC header.
	return kFlexfecPacketMaskSizes[1];
	} else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet &&
	(packet_mask[5] & 0x03) == 0) {
	// Packet mask is 48 bits long, with bits 46 and 47 clear.
	// It can be used as is.
	return kFlexfecPacketMaskSizes[1];
	} else if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet) {
	// Packet mask is 48 bits long, with at least one of bits 46 and 47 set.
	// We must expand it with zeros.
	return kFlexfecPacketMaskSizes[2];
	}
	RTC_NOTREACHED() << "Incorrect packet mask size: " << packet_mask_size << ".";
	return kFlexfecPacketMaskSizes[2];
	}

	size_t FlexfecHeaderWriter::FecHeaderSize(size_t packet_mask_size) const {
	return FlexfecHeaderSize(packet_mask_size);
	}

	// This function adapts the precomputed ULPFEC packet masks to the
	// FlexFEC header standard. Note that the header size is computed by
	// FecHeaderSize(), so in this function we can be sure that we are
	// writing in space that is intended for the header.
	//
	// TODO(brandtr): Update this function when we support offset-based masks,
	// retransmissions, and protecting multiple SSRCs.
	void FlexfecHeaderWriter::FinalizeFecHeader(
	uint32_t media_ssrc,
	uint16_t seq_num_base,
	const uint8_t* packet_mask,
	size_t packet_mask_size,
	ForwardErrorCorrection::Packet* fec_packet) const {
	fec_packet->data[0] &= 0x7f; // Clear R bit.
	fec_packet->data[0] &= 0xbf; // Clear F bit.
	ByteWriter<uint8_t>::WriteBigEndian(&fec_packet->data[8], kSsrcCount);
	ByteWriter<uint32_t, 3>::WriteBigEndian(&fec_packet->data[9], kReservedBits);
	ByteWriter<uint32_t>::WriteBigEndian(&fec_packet->data[12], media_ssrc);
	ByteWriter<uint16_t>::WriteBigEndian(&fec_packet->data[16], seq_num_base);
	// Adapt ULPFEC packet mask to FlexFEC header.
	//
	// We treat the mask parts as unsigned integers with host order endianness
	// in order to simplify the bit shifting between bytes.
	uint8_t* const written_packet_mask = fec_packet->data + kPacketMaskOffset;
	if (packet_mask_size == kUlpfecPacketMaskSizeLBitSet) {
	// The packet mask is 48 bits long.
	uint16_t tmp_mask_part0 =
	ByteReader<uint16_t>::ReadBigEndian(&packet_mask[0]);
	uint32_t tmp_mask_part1 =
	ByteReader<uint32_t>::ReadBigEndian(&packet_mask[2]);

	tmp_mask_part0 >>= 1; // Shift, thus clearing K-bit 0.
	ByteWriter<uint16_t>::WriteBigEndian(&written_packet_mask[0],
	tmp_mask_part0);
	tmp_mask_part1 >>= 2; // Shift, thus clearing K-bit 1 and bit 15.
	ByteWriter<uint32_t>::WriteBigEndian(&written_packet_mask[2],
	tmp_mask_part1);
	bool bit15 = (packet_mask[1] & 0x01) != 0;
	if (bit15)
	written_packet_mask[2] \|= 0x40; // Set bit 15.
	bool bit46 = (packet_mask[5] & 0x02) != 0;
	bool bit47 = (packet_mask[5] & 0x01) != 0;
	if (!bit46 && !bit47) {
	written_packet_mask[2] \|= 0x80; // Set K-bit 1.
	} else {
	memset(&written_packet_mask[6], 0, 8); // Clear all trailing bits.
	written_packet_mask[6] \|= 0x80; // Set K-bit 2.
	if (bit46)
	written_packet_mask[6] \|= 0x40; // Set bit 46.
	if (bit47)
	written_packet_mask[6] \|= 0x20; // Set bit 47.
	}
	} else if (packet_mask_size == kUlpfecPacketMaskSizeLBitClear) {
	// The packet mask is 16 bits long.
	uint16_t tmp_mask_part0 =
	ByteReader<uint16_t>::ReadBigEndian(&packet_mask[0]);

	tmp_mask_part0 >>= 1; // Shift, thus clearing K-bit 0.
	ByteWriter<uint16_t>::WriteBigEndian(&written_packet_mask[0],
	tmp_mask_part0);
	bool bit15 = (packet_mask[1] & 0x01) != 0;
	if (!bit15) {
	written_packet_mask[0] \|= 0x80; // Set K-bit 0.
	} else {
	memset(&written_packet_mask[2], 0U, 4); // Clear all trailing bits.
	written_packet_mask[2] \|= 0x80; // Set K-bit 1.
	written_packet_mask[2] \|= 0x40; // Set bit 15.
	}
	} else {
	RTC_NOTREACHED() << "Incorrect packet mask size: " << packet_mask_size
	<< ".";
	}
	}

	} // namespace webrtc