| /* |
| * 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 |