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/*
* 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 "webrtc/modules/rtp_rtcp/source/flexfec_header_reader_writer.h"
#include <string.h>
#include <utility>
#include "webrtc/modules/rtp_rtcp/source/byte_io.h"
#include "webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h"
#include "webrtc/rtc_base/checks.h"
#include "webrtc/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) {
LOG(LS_WARNING) << "Discarding truncated FlexFEC packet.";
return false;
}
bool r_bit = (fec_packet->pkt->data[0] & 0x80) != 0;
if (r_bit) {
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) {
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) {
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]) {
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]) {
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 {
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