Increase FEC receiver's protected packet queue size.

- The FEC receiver tracks maximum of 48 media packets at a time, and packet reordering can delay the FEC packet from its protected media packets by more than 48 sequences.
- Such FEC packets do not get purged until much later when newer FEC packets with much higher sequence mark them as old.
- Until that happens, they sit in the receiver queue, wasting CPU cycles.
- If the receiver maintains a larger queue size for the media packets, it increases possibility of having all media packets in the queue, thereby organically purging the FEC packet.
- More importantly, this also increases the efficacy of FEC decode for such packet, since media packets now remain relevant for longer and aid in lost packet recovery.

Bug: webrtc:12656
Change-Id: Id0058df9a23ea31839decf2c37e0670a54c947fc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215882
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33989}

modules/rtp_rtcp/source/flexfec_header_reader_writer.cc

@@ -25,6 +25,11 @@ 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;
 
@@ -72,7 +77,7 @@ size_t FlexfecHeaderSize(size_t packet_mask_size) {
 }  // namespace
 
 FlexfecHeaderReader::FlexfecHeaderReader()
-    : FecHeaderReader(kMaxMediaPackets, kMaxFecPackets) {}
+    : FecHeaderReader(kMaxTrackedMediaPackets, kMaxFecPackets) {}
 
 FlexfecHeaderReader::~FlexfecHeaderReader() = default;
 

modules/rtp_rtcp/source/flexfec_receiver_unittest.cc

@@ -374,7 +374,8 @@ TEST_F(FlexfecReceiverTest, RecoversFrom50PercentLoss) {
 TEST_F(FlexfecReceiverTest, DelayedFecPacketDoesHelp) {
   // These values need to be updated if the underlying erasure code
   // implementation changes.
-  const size_t kNumFrames = 48;
+  // Delay FEC packet by maximum number of media packets tracked by receiver.
+  const size_t kNumFrames = 192;
   const size_t kNumMediaPacketsPerFrame = 1;
   const size_t kNumFecPackets = 1;
 
@@ -412,14 +413,16 @@ TEST_F(FlexfecReceiverTest, DelayedFecPacketDoesHelp) {
 TEST_F(FlexfecReceiverTest, TooDelayedFecPacketDoesNotHelp) {
   // These values need to be updated if the underlying erasure code
   // implementation changes.
-  const size_t kNumFrames = 49;
+  // Delay FEC packet by one more than maximum number of media packets
+  // tracked by receiver.
+  const size_t kNumFrames = 193;
   const size_t kNumMediaPacketsPerFrame = 1;
   const size_t kNumFecPackets = 1;
 
   PacketList media_packets;
   PacketizeFrame(kNumMediaPacketsPerFrame, 0, &media_packets);
   PacketizeFrame(kNumMediaPacketsPerFrame, 1, &media_packets);
-  // Protect two first frames.
+  // Protect first two frames.
   std::list<Packet*> fec_packets = EncodeFec(media_packets, kNumFecPackets);
   for (size_t i = 2; i < kNumFrames; ++i) {
     PacketizeFrame(kNumMediaPacketsPerFrame, i, &media_packets);
@@ -667,11 +670,11 @@ TEST_F(FlexfecReceiverTest, DoesNotDecodeWrappedMediaSequenceUsingOldFec) {
     PacketizeFrame(kNumMediaPacketsPerFrame, i, &media_packets);
   }
 
-  // Receive first (|kFirstFrameNumMediaPackets| + 48) media packets.
+  // Receive first (|kFirstFrameNumMediaPackets| + 192) media packets.
   // Simulate an old FEC packet by separating it from its encoded media
-  // packets by at least 48 packets.
+  // packets by at least 192 packets.
   auto media_it = media_packets.begin();
-  for (size_t i = 0; i < (kFirstFrameNumMediaPackets + 48); i++) {
+  for (size_t i = 0; i < (kFirstFrameNumMediaPackets + 192); i++) {
     if (i == 1) {
       // Drop the second packet of the first frame.
       media_it++;
@@ -682,7 +685,7 @@ TEST_F(FlexfecReceiverTest, DoesNotDecodeWrappedMediaSequenceUsingOldFec) {
 
   // Receive FEC packet. Although a protected packet was dropped,
   // expect no recovery callback since it is delayed from first frame
-  // by more than 48 packets.
+  // by more than 192 packets.
   auto fec_it = fec_packets.begin();
   std::unique_ptr<Packet> fec_packet_with_rtp_header =
       packet_generator_.BuildFlexfecPacket(**fec_it);

modules/rtp_rtcp/source/ulpfec_header_reader_writer.cc

@@ -24,6 +24,11 @@ namespace {
 // Maximum number of media packets that can be protected in one batch.
 constexpr size_t kMaxMediaPackets = 48;
 
+// 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;
 
@@ -51,7 +56,7 @@ size_t UlpfecHeaderSize(size_t packet_mask_size) {
 }  // namespace
 
 UlpfecHeaderReader::UlpfecHeaderReader()
-    : FecHeaderReader(kMaxMediaPackets, kMaxFecPackets) {}
+    : FecHeaderReader(kMaxTrackedMediaPackets, kMaxFecPackets) {}
 
 UlpfecHeaderReader::~UlpfecHeaderReader() = default;
 

modules/rtp_rtcp/source/ulpfec_receiver_unittest.cc

@@ -392,7 +392,7 @@ TEST_F(UlpfecReceiverTest, PacketNotDroppedTooEarly) {
   delayed_fec = fec_packets.front();
 
   // Fill the FEC decoder. No packets should be dropped.
-  const size_t kNumMediaPacketsBatch2 = 46;
+  const size_t kNumMediaPacketsBatch2 = 191;
   std::list<AugmentedPacket*> augmented_media_packets_batch2;
   ForwardErrorCorrection::PacketList media_packets_batch2;
   for (size_t i = 0; i < kNumMediaPacketsBatch2; ++i) {
@@ -431,7 +431,7 @@ TEST_F(UlpfecReceiverTest, PacketDroppedWhenTooOld) {
   delayed_fec = fec_packets.front();
 
   // Fill the FEC decoder and force the last packet to be dropped.
-  const size_t kNumMediaPacketsBatch2 = 48;
+  const size_t kNumMediaPacketsBatch2 = 192;
   std::list<AugmentedPacket*> augmented_media_packets_batch2;
   ForwardErrorCorrection::PacketList media_packets_batch2;
   for (size_t i = 0; i < kNumMediaPacketsBatch2; ++i) {