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/*
* Copyright (c) 2023 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 media packets tracked by FEC decoder.
// Maintain a sufficiently larger tracking window than `kMaxMediaPackets`
// to account for packet reordering in pacer/ network.
constexpr size_t kMaxTrackedMediaPackets = 4 * kMaxMediaPackets;
// 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 = 8;
// Size (in bytes) of part of header which is stream specific.
constexpr size_t kStreamSpecificHeaderSize = 2;
// 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]};
// 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(kMaxTrackedMediaPackets, kMaxFecPackets) {}
FlexfecHeaderReader::~FlexfecHeaderReader() = default;
// TODO(brandtr): Update this function when we support flexible masks,
// and retransmissions.
bool FlexfecHeaderReader::ReadFecHeader(
ForwardErrorCorrection::ReceivedFecPacket* fec_packet) const {
// Protected ssrcs should already be populated from RTP header.
if (fec_packet->protected_streams.empty()) {
RTC_LOG(LS_WARNING)
<< "Discarding FlexFEC packet with no protected sources.";
return false;
}
if (fec_packet->pkt->data.size() <=
kBaseHeaderSize + kStreamSpecificHeaderSize) {
RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
return false;
}
uint8_t* const data = fec_packet->pkt->data.MutableData();
bool r_bit = (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 = (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;
}
// First seq_num will be in byte index 8
// (See FEC header schematic in flexfec_header_reader_writer.h.)
size_t byte_index = 8;
for (size_t i = 0; i < fec_packet->protected_streams.size(); ++i) {
if (fec_packet->pkt->data.size() < byte_index + kStreamSpecificHeaderSize) {
RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
return false;
}
fec_packet->protected_streams[i].seq_num_base =
ByteReader<uint16_t>::ReadBigEndian(&data[byte_index]);
byte_index += kStreamSpecificHeaderSize;
// 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->data.size() <
(byte_index + kFlexfecPacketMaskSizes[0])) {
RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
return false;
}
fec_packet->protected_streams[i].packet_mask_offset = byte_index;
bool k_bit0 = (data[byte_index] & 0x80) != 0;
uint16_t mask_part0 =
ByteReader<uint16_t>::ReadBigEndian(&data[byte_index]);
// Shift away K-bit 0, implicitly clearing the last bit.
mask_part0 <<= 1;
ByteWriter<uint16_t>::WriteBigEndian(&data[byte_index], mask_part0);
byte_index += kFlexfecPacketMaskSizes[0];
if (k_bit0) {
// The first K-bit is set, and the packet mask is thus only 2 bytes long.
// We have finished reading the properties for current ssrc.
fec_packet->protected_streams[i].packet_mask_size =
kFlexfecPacketMaskSizes[0];
} else {
if (fec_packet->pkt->data.size() <
(byte_index + kFlexfecPacketMaskSizes[1] -
kFlexfecPacketMaskSizes[0])) {
return false;
}
bool k_bit1 = (data[byte_index] & 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 = (data[byte_index] >> 6) & 0x01;
data[byte_index - 1] |= bit15;
uint32_t mask_part1 =
ByteReader<uint32_t>::ReadBigEndian(&data[byte_index]);
// Shift away K-bit 1 and bit 15, implicitly clearing the last two bits.
mask_part1 <<= 2;
ByteWriter<uint32_t>::WriteBigEndian(&data[byte_index], mask_part1);
byte_index += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
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 finished reading the properties
// for current ssrc.
fec_packet->protected_streams[i].packet_mask_size =
kFlexfecPacketMaskSizes[1];
} else {
if (fec_packet->pkt->data.size() <
(byte_index + kFlexfecPacketMaskSizes[2] -
kFlexfecPacketMaskSizes[1])) {
RTC_LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
return false;
}
fec_packet->protected_streams[i].packet_mask_size =
kFlexfecPacketMaskSizes[2];
// 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 remaining part of the packet mask two steps to
// the left. This corresponds to the (in total) two K-bits, which
// have been removed.
uint8_t tail_bits = (data[byte_index] >> 6) & 0x03;
data[byte_index - 1] |= tail_bits;
uint64_t mask_part2 =
ByteReader<uint64_t>::ReadBigEndian(&data[byte_index]);
// Shift away bit 46, and bit 47, which were copied to the previous
// part of the mask, implicitly clearing the last two bits.
mask_part2 <<= 2;
ByteWriter<uint64_t>::WriteBigEndian(&data[byte_index], mask_part2);
byte_index += kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1];
}
}
}
fec_packet->fec_header_size = byte_index;
// In FlexFEC, all media packets are protected in their entirety.
fec_packet->protection_length =
fec_packet->pkt->data.size() - 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_DCHECK_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
// and retransmissions.
void FlexfecHeaderWriter::FinalizeFecHeader(
rtc::ArrayView<const ProtectedStream> protected_streams,
ForwardErrorCorrection::Packet& fec_packet) const {
uint8_t* data = fec_packet.data.MutableData();
*data &= 0x7f; // Clear R bit.
*data &= 0xbf; // Clear F bit.
// First seq_num will be in byte index 8
// (See FEC header schematic in flexfec_header_reader_writer.h.)
uint8_t* write_at = data + 8;
for (const ProtectedStream& protected_stream : protected_streams) {
ByteWriter<uint16_t>::WriteBigEndian(write_at,
protected_stream.seq_num_base);
write_at += kStreamSpecificHeaderSize;
// 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.
if (protected_stream.packet_mask.size() == kUlpfecPacketMaskSizeLBitSet) {
// The packet mask is 48 bits long.
uint16_t tmp_mask_part0 =
ByteReader<uint16_t>::ReadBigEndian(&protected_stream.packet_mask[0]);
uint32_t tmp_mask_part1 =
ByteReader<uint32_t>::ReadBigEndian(&protected_stream.packet_mask[2]);
tmp_mask_part0 >>= 1; // Shift, thus clearing K-bit 0.
ByteWriter<uint16_t>::WriteBigEndian(write_at, tmp_mask_part0);
write_at += kFlexfecPacketMaskSizes[0];
tmp_mask_part1 >>= 2; // Shift, thus clearing K-bit 1 and bit 15.
ByteWriter<uint32_t>::WriteBigEndian(write_at, tmp_mask_part1);
bool bit15 = (protected_stream.packet_mask[1] & 0x01) != 0;
if (bit15)
*write_at |= 0x40; // Set bit 15.
bool bit46 = (protected_stream.packet_mask[5] & 0x02) != 0;
bool bit47 = (protected_stream.packet_mask[5] & 0x01) != 0;
if (!bit46 && !bit47) {
*write_at |= 0x80; // Set K-bit 1.
write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
} else {
write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
// Clear all trailing bits.
memset(write_at, 0,
kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1]);
if (bit46)
*write_at |= 0x80; // Set bit 46.
if (bit47)
*write_at |= 0x40; // Set bit 47.
write_at += kFlexfecPacketMaskSizes[2] - kFlexfecPacketMaskSizes[1];
}
} else if (protected_stream.packet_mask.size() ==
kUlpfecPacketMaskSizeLBitClear) {
// The packet mask is 16 bits long.
uint16_t tmp_mask_part0 =
ByteReader<uint16_t>::ReadBigEndian(&protected_stream.packet_mask[0]);
tmp_mask_part0 >>= 1; // Shift, thus clearing K-bit 0.
ByteWriter<uint16_t>::WriteBigEndian(write_at, tmp_mask_part0);
bool bit15 = (protected_stream.packet_mask[1] & 0x01) != 0;
if (!bit15) {
*write_at |= 0x80; // Set K-bit 0.
write_at += kFlexfecPacketMaskSizes[0];
} else {
write_at += kFlexfecPacketMaskSizes[0];
// Clear all trailing bits.
memset(write_at, 0U,
kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0]);
*write_at |= 0x80; // Set K-bit 1.
*write_at |= 0x40; // Set bit 15.
write_at += kFlexfecPacketMaskSizes[1] - kFlexfecPacketMaskSizes[0];
}
} else {
RTC_DCHECK_NOTREACHED() << "Incorrect packet mask size: "
<< protected_stream.packet_mask.size() << ".";
}
}
}
} // namespace webrtc