1 files changed, 24 insertions, 7 deletions

diff --git a/modules/rtp_rtcp/source/forward_error_correction.cc b/modules/rtp_rtcp/source/forward_error_correction.cc
index 56eabc8a7f..da8025d3db 100644
--- a/modules/rtp_rtcp/source/forward_error_correction.cc
+++ b/modules/rtp_rtcp/source/forward_error_correction.cc
@@ -31,6 +31,8 @@ namespace webrtc {
 namespace {
 // Transport header size in bytes. Assume UDP/IPv4 as a reasonable minimum.
 constexpr size_t kTransportOverhead = 28;
+
+constexpr uint16_t kOldSequenceThreshold = 0x3fff;
 }  // namespace
 
 ForwardErrorCorrection::Packet::Packet() : data(0), ref_count_(0) {}
@@ -508,9 +510,6 @@ void ForwardErrorCorrection::InsertPacket(
   // This is important for keeping |received_fec_packets_| sorted, and may
   // also reduce the possibility of incorrect decoding due to sequence number
   // wrap-around.
-  // TODO(marpan/holmer): We should be able to improve detection/discarding of
-  // old FEC packets based on timestamp information or better sequence number
-  // thresholding (e.g., to distinguish between wrap-around and reordering).
   if (!received_fec_packets_.empty() &&
       received_packet.ssrc == received_fec_packets_.front()->ssrc) {
     // It only makes sense to detect wrap-around when |received_packet|
@@ -521,7 +520,7 @@ void ForwardErrorCorrection::InsertPacket(
     auto it = received_fec_packets_.begin();
     while (it != received_fec_packets_.end()) {
       uint16_t seq_num_diff = MinDiff(received_packet.seq_num, (*it)->seq_num);
-      if (seq_num_diff > 0x3fff) {
+      if (seq_num_diff > kOldSequenceThreshold) {
         it = received_fec_packets_.erase(it);
       } else {
         // No need to keep iterating, since |received_fec_packets_| is sorted.
@@ -698,9 +697,10 @@ void ForwardErrorCorrection::AttemptRecovery(
       // this may allow additional packets to be recovered.
       // Restart for first FEC packet.
       fec_packet_it = received_fec_packets_.begin();
-    } else if (packets_missing == 0) {
-      // Either all protected packets arrived or have been recovered. We can
-      // discard this FEC packet.
+    } else if (packets_missing == 0 ||
+               IsOldFecPacket(**fec_packet_it, recovered_packets)) {
+      // Either all protected packets arrived or have been recovered, or the FEC
+      // packet is old. We can discard this FEC packet.
       fec_packet_it = received_fec_packets_.erase(fec_packet_it);
     } else {
       fec_packet_it++;
@@ -731,6 +731,23 @@ void ForwardErrorCorrection::DiscardOldRecoveredPackets(
   RTC_DCHECK_LE(recovered_packets->size(), max_media_packets);
 }
 
+bool ForwardErrorCorrection::IsOldFecPacket(
+    const ReceivedFecPacket& fec_packet,
+    const RecoveredPacketList* recovered_packets) {
+  if (recovered_packets->empty()) {
+    return false;
+  }
+
+  const uint16_t back_recovered_seq_num = recovered_packets->back()->seq_num;
+  const uint16_t last_protected_seq_num =
+      fec_packet.protected_packets.back()->seq_num;
+
+  // FEC packet is old if its last protected sequence number is much
+  // older than the latest protected sequence number received.
+  return (MinDiff(back_recovered_seq_num, last_protected_seq_num) >
+          kOldSequenceThreshold);
+}
+
 uint16_t ForwardErrorCorrection::ParseSequenceNumber(const uint8_t* packet) {
   return (packet[2] << 8) + packet[3];
 }