Only compare sequence numbers from the same SSRC in ForwardErrorCorrection.

webrtc/modules/rtp_rtcp/source/forward_error_correction.cc

Index: webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
diff --git a/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc b/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
index 8c35f4000f97a848f3d9de5bc382b9b0d1b3ada0..193629b0b9a270bf70819afc60eb323b70cb901e 100644
--- a/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
+++ b/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
@@ -54,6 +54,11 @@ template <typename S, typename T>
 bool ForwardErrorCorrection::SortablePacket::LessThan::operator() (
     const S& first,
     const T& second) {
+  // TODO(brandtr): Try to add a DCHECK here, verifying that
+  // first->ssrc == second->ssrc. Currently, it is hard to do so
+  // because we are unable to reliably set the ssrc member on the
+  // ProtectedPacket's. These always belong to the media SSRC, but
+  // we may not know the media SSRC when they are created.
   return IsNewerSequenceNumber(second->seq_num, first->seq_num);
 }
 
@@ -337,6 +342,7 @@ void ForwardErrorCorrection::InsertMediaPacket(
     ReceivedPacket* received_packet) {
   // Search for duplicate packets.
   for (const auto& recovered_packet : *recovered_packets) {
+    RTC_DCHECK_EQ(received_packet->ssrc, recovered_packet->ssrc);
     if (received_packet->seq_num == recovered_packet->seq_num) {
       // Duplicate packet, no need to add to list.
       // Delete duplicate media packet data.
@@ -349,6 +355,7 @@ void ForwardErrorCorrection::InsertMediaPacket(
   recovered_packet->was_recovered = false;
   // This media packet has already been passed on.
   recovered_packet->returned = true;
+  recovered_packet->ssrc = received_packet->ssrc;
   recovered_packet->seq_num = received_packet->seq_num;
   recovered_packet->pkt = received_packet->pkt;
   recovered_packet->pkt->length = received_packet->pkt->length;
@@ -380,6 +387,7 @@ void ForwardErrorCorrection::InsertFecPacket(
     ReceivedPacket* received_packet) {
   // Check for duplicate.
   for (const auto& existing_fec_packet : received_fec_packets_) {
+    RTC_DCHECK_EQ(received_packet->ssrc, existing_fec_packet->ssrc);
     if (received_packet->seq_num == existing_fec_packet->seq_num) {
       // Delete duplicate FEC packet data.
       received_packet->pkt = nullptr;
@@ -388,8 +396,8 @@ void ForwardErrorCorrection::InsertFecPacket(
   }
   std::unique_ptr<ReceivedFecPacket> fec_packet(new ReceivedFecPacket());
   fec_packet->pkt = received_packet->pkt;
-  fec_packet->seq_num = received_packet->seq_num;
   fec_packet->ssrc = received_packet->ssrc;
+  fec_packet->seq_num = received_packet->seq_num;
   // Parse ULPFEC/FlexFEC header specific info.
   bool ret = fec_header_reader_->ReadFecHeader(fec_packet.get());
   if (!ret) {
@@ -419,11 +427,6 @@ void ForwardErrorCorrection::InsertFecPacket(
     AssignRecoveredPackets(recovered_packets, fec_packet.get());
     // TODO(holmer): Consider replacing this with a binary search for the right
     // position, and then just insert the new packet. Would get rid of the sort.
-    //
-    // For correct decoding, |received_fec_packets_| does not necessarily
-    // need to be sorted by sequence number (see decoding algorithm in
-    // AttemptRecover()). By keeping it sorted we try to recover the
-    // oldest lost packets first, however.
     received_fec_packets_.push_back(std::move(fec_packet));
     received_fec_packets_.sort(SortablePacket::LessThan());
     const size_t max_fec_packets = fec_header_reader_->MaxFecPackets();
@@ -467,19 +470,31 @@ void ForwardErrorCorrection::InsertPackets(
   while (!received_packets->empty()) {
     ReceivedPacket* received_packet = received_packets->front().get();
 
-    // Check for discarding oldest FEC packet, to avoid wrong FEC decoding from
-    // sequence number wrap-around. Detection of old FEC packet is based on
-    // sequence number difference of received packet and oldest packet in FEC
-    // packet list.
+    // Discard old FEC packets such that the sequence numbers in
+    // |received_fec_packets_| span at most 1/2 of the sequence number space.
+    // 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()) {
-      uint16_t seq_num_diff =
-          abs(static_cast<int>(received_packet->seq_num) -
-              static_cast<int>(received_fec_packets_.front()->seq_num));
-      if (seq_num_diff > 0x3fff) {
-        received_fec_packets_.pop_front();
+    if (!received_fec_packets_.empty() &&
+        received_packet->ssrc == received_fec_packets_.front()->ssrc) {
+      // It only makes sense to detect wrap-around when |received_packet|
+      // and |front_received_fec_packet| belong to the same sequence number
+      // space, i.e., the same SSRC. This happens when |received_packet|
+      // is a FEC packet, or if |received_packet| is a media packet and
+      // RED+ULPFEC is used.
+      auto it = received_fec_packets_.begin();
+      while (it != received_fec_packets_.end()) {
+        uint16_t seq_num_diff = abs(static_cast<int>(received_packet->seq_num) -
+                                    static_cast<int>((*it)->seq_num));
+        if (seq_num_diff > 0x3fff) {
+          it = received_fec_packets_.erase(it);
+        } else {
+          // No need to keep iterating, since |received_fec_packets_| is sorted.
+          break;
+        }
       }
     }
 
@@ -549,6 +564,7 @@ bool ForwardErrorCorrection::FinishPacketRecovery(
   // Set the SSRC field.
   ByteWriter<uint32_t>::WriteBigEndian(&recovered_packet->pkt->data[8],
                                        fec_packet.protected_ssrc);
+  recovered_packet->ssrc = fec_packet.protected_ssrc;
   return true;
 }
 
@@ -684,21 +700,34 @@ uint32_t ForwardErrorCorrection::ParseSsrc(uint8_t* packet) {
 int ForwardErrorCorrection::DecodeFec(
     ReceivedPacketList* received_packets,
     RecoveredPacketList* recovered_packets) {
-  // TODO(marpan/ajm): can we check for multiple ULP headers, and return an
-  // error?
+  RTC_DCHECK(received_packets);
+  RTC_DCHECK(recovered_packets);
+
   const size_t max_media_packets = fec_header_reader_->MaxMediaPackets();
   if (recovered_packets->size() == max_media_packets) {
-    const unsigned int seq_num_diff =
-        abs(static_cast<int>(received_packets->front()->seq_num) -
-            static_cast<int>(recovered_packets->back()->seq_num));
-    if (seq_num_diff > max_media_packets) {
-      // A big gap in sequence numbers. The old recovered packets
-      // are now useless, so it's safe to do a reset.
-      ResetState(recovered_packets);
+    const RecoveredPacket* back_recovered_packet =
+        recovered_packets->back().get();
+    for (const auto& received_packet : *received_packets) {
+      if (received_packet->ssrc == back_recovered_packet->ssrc) {
+        const unsigned int seq_num_diff =
+            abs(static_cast<int>(received_packet->seq_num) -
+                static_cast<int>(back_recovered_packet->seq_num));
+        if (seq_num_diff > max_media_packets) {
+          // A big gap in sequence numbers. The old recovered packets
+          // are now useless, so it's safe to do a reset.
+          LOG(LS_INFO) << "Big gap in media/ULPFEC sequence numbers. No need "
+                          "to keep the old packets in the FEC buffers, thus "
+                          "resetting them.";
+          ResetState(recovered_packets);
+          break;
+        }
+      }
     }
   }
+
   InsertPackets(received_packets, recovered_packets);
   AttemptRecovery(recovered_packets);
+
   return 0;
 }