Add FlexFEC header formatters. (pt. 5)

webrtc/modules/rtp_rtcp/source/flexfec_header_reader_writer.cc

+/*
+ *  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/base/checks.h"
+#include "webrtc/base/logging.h"
+#include "webrtc/modules/rtp_rtcp/source/byte_io.h"
+#include "webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.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 f_bit = (fec_packet->pkt->data[0] & 0x80) != 0;
+  if (f_bit) {
+    LOG(LS_INFO) << "FlexFEC packet with inflexible generator matrix. We do "
+                    "not yet support this, thus discarding packet.";
+    return false;
+  }
+  bool r_bit = (fec_packet->pkt->data[0] & 0x40) != 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;
+  }
+  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;

[stefan-webrtc, 2016/09/30 08:22:43]

Seems like a waste that k_bit2 has no meaning. We should look into that.

+      }
+      // 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 F bit.
+  fec_packet->data[0] &= 0xbf;  // Clear R 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