Generalize FEC header formatting. (pt. 4)

webrtc/modules/rtp_rtcp/source/ulpfec_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/ulpfec_header_reader_writer.h"
+
+#include <utility>
+
+#include "webrtc/base/checks.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;
+
+// Maximum number of FEC packets stored inside ForwardErrorCorrection.
+constexpr size_t kMaxFecPackets = kMaxMediaPackets;
+
+// FEC Level 0 header size in bytes.
+constexpr size_t kFecLevel0HeaderSize = 10;
+
+// FEC Level 1 (ULP) header size in bytes (L bit is set).
+constexpr size_t kFecLevel1HeaderSizeLBitSet = 2 + kUlpfecPacketMaskSizeLBitSet;
+
+// FEC Level 1 (ULP) header size in bytes (L bit is cleared).
+constexpr size_t kFecLevel1HeaderSizeLBitClear =
+    2 + kUlpfecPacketMaskSizeLBitClear;
+
+constexpr size_t kPacketMaskOffset = kFecLevel0HeaderSize + 2;
+
+size_t UlpfecHeaderSize(size_t packet_mask_size) {
+  RTC_DCHECK_LE(packet_mask_size, kUlpfecPacketMaskSizeLBitSet);
+  if (packet_mask_size <= kUlpfecPacketMaskSizeLBitClear) {
+    return kFecLevel0HeaderSize + kFecLevel1HeaderSizeLBitClear;
+  } else {
+    return kFecLevel0HeaderSize + kFecLevel1HeaderSizeLBitSet;
+  }
+}
+
+}  // namespace
+
+UlpfecHeaderReader::UlpfecHeaderReader()
+    : FecHeaderReader(kMaxMediaPackets, kMaxFecPackets) {}
+
+UlpfecHeaderReader::~UlpfecHeaderReader() = default;
+
+bool UlpfecHeaderReader::ReadFecHeader(
+    ForwardErrorCorrection::ReceivedFecPacket* fec_packet) const {
+  bool l_bit = (fec_packet->pkt->data[0] & 0x40) != 0u;
+  size_t packet_mask_size =
+      l_bit ? kUlpfecPacketMaskSizeLBitSet : kUlpfecPacketMaskSizeLBitClear;
+  fec_packet->fec_header_size = UlpfecHeaderSize(packet_mask_size);
+  uint16_t seq_num_base =
+      ByteReader<uint16_t>::ReadBigEndian(&fec_packet->pkt->data[2]);
+  fec_packet->protected_ssrc = fec_packet->ssrc;  // Due to RED.
+  fec_packet->seq_num_base = seq_num_base;
+  fec_packet->packet_mask_offset = kPacketMaskOffset;
+  fec_packet->packet_mask_size = packet_mask_size;
+  fec_packet->protection_length =
+      ByteReader<uint16_t>::ReadBigEndian(&fec_packet->pkt->data[10]);
+
+  // Store length recovery field in temporary location in header.
+  // This makes the header "compatible" with the corresponding
+  // FlexFEC location of the length recovery field, thus simplifying
+  // the XORing operations.
+  memcpy(&fec_packet->pkt->data[2], &fec_packet->pkt->data[8], 2);
+
+  return true;
+}
+
+UlpfecHeaderWriter::UlpfecHeaderWriter()
+    : FecHeaderWriter(kMaxMediaPackets,
+                      kMaxFecPackets,
+                      kFecLevel0HeaderSize + kFecLevel1HeaderSizeLBitSet) {}
+
+UlpfecHeaderWriter::~UlpfecHeaderWriter() = default;
+
+// TODO(brandtr): Consider updating this implementation (which actually
+// returns a bound on the sequence number spread), if logic is added to
+// UlpfecHeaderWriter::FinalizeFecHeader to truncate packet masks which end
+// in a string of zeroes. (Similar to how it is done in the FlexFEC case.)
+size_t UlpfecHeaderWriter::MinPacketMaskSize(const uint8_t* packet_mask,
+                                             size_t packet_mask_size) const {
+  return packet_mask_size;
+}
+
+size_t UlpfecHeaderWriter::FecHeaderSize(size_t packet_mask_size) const {
+  return UlpfecHeaderSize(packet_mask_size);
+}
+
+void UlpfecHeaderWriter::FinalizeFecHeader(
+    uint16_t seq_num_base,
+    const uint8_t* packet_mask,
+    size_t packet_mask_size,
+    ForwardErrorCorrection::Packet* fec_packet) const {
+  // Set E bit to zero.
+  fec_packet->data[0] &= 0x7f;
+  // Set L bit based on packet mask size. (Note that the packet mask
+  // can only take on two discrete values.)
+  bool l_bit = (packet_mask_size == kUlpfecPacketMaskSizeLBitSet);
+  if (l_bit) {
+    fec_packet->data[0] |= 0x40;  // Set the L bit.
+  } else {
+    RTC_DCHECK_EQ(packet_mask_size, kUlpfecPacketMaskSizeLBitClear);
+    fec_packet->data[0] &= 0xbf;  // Clear the L bit.
+  }
+  // Copy length recovery field from temporary location.
+  memcpy(&fec_packet->data[8], &fec_packet->data[2], 2);
+  // Write sequence number base.
+  ByteWriter<uint16_t>::WriteBigEndian(&fec_packet->data[2], seq_num_base);
+  // Protection length is set to entire packet. (This is not
+  // required in general.)
+  const size_t fec_header_size = FecHeaderSize(packet_mask_size);
+  ByteWriter<uint16_t>::WriteBigEndian(&fec_packet->data[10],
+                                       fec_packet->length - fec_header_size);
+  // Copy the packet mask.
+  memcpy(&fec_packet->data[12], packet_mask, packet_mask_size);
+}
+
+}  // namespace webrtc