diff --git a/webrtc/modules/remote_bitrate_estimator/test/packet_sender.cc b/webrtc/modules/remote_bitrate_estimator/test/packet_sender.cc
index 80a9710f7..3e0073a41 100644
--- a/webrtc/modules/remote_bitrate_estimator/test/packet_sender.cc
+++ b/webrtc/modules/remote_bitrate_estimator/test/packet_sender.cc
@@ -276,16 +276,25 @@ void TcpSender::RunFor(int64_t time_ms, Packets* in_out) {
     UpdateCongestionControl(fb);
     SendPackets(in_out);
   }
+
+  for (auto it = in_flight_.begin(); it != in_flight_.end();) {
+    if (it->time_ms < now_ms_ - 1000)
+      in_flight_.erase(it++);
+    else
+      ++it;
+  }
+
   SendPackets(in_out);
   now_ms_ += time_ms;
 }
 
 void TcpSender::SendPackets(Packets* in_out) {
   int cwnd = ceil(cwnd_);
-  int packets_to_send = std::max(cwnd - in_flight_, 0);
+  int packets_to_send = std::max(cwnd - static_cast<int>(in_flight_.size()), 0);
   if (packets_to_send > 0) {
     Packets generated = GeneratePackets(packets_to_send);
-    in_flight_ += static_cast<int>(generated.size());
+    for (Packet* packet : generated)
+      in_flight_.insert(InFlight(*static_cast<MediaPacket*>(packet)));
     in_out->merge(generated, DereferencingComparator<Packet>);
   }
 }
@@ -296,10 +305,12 @@ void TcpSender::UpdateCongestionControl(const FeedbackPacket* fb) {
   ack_received_ = true;
 
   uint16_t expected = tcp_fb->acked_packets().back() - last_acked_seq_num_;
-  in_flight_ -= expected;
   uint16_t missing =
       expected - static_cast<uint16_t>(tcp_fb->acked_packets().size());
 
+  for (uint16_t ack_seq_num : tcp_fb->acked_packets())
+    in_flight_.erase(InFlight(ack_seq_num, now_ms_));
+
   if (missing > 0) {
     cwnd_ /= 2.0f;
     in_slow_start_ = false;
diff --git a/webrtc/modules/remote_bitrate_estimator/test/packet_sender.h b/webrtc/modules/remote_bitrate_estimator/test/packet_sender.h
index 4bdd5392c..0c579ed09 100644
--- a/webrtc/modules/remote_bitrate_estimator/test/packet_sender.h
+++ b/webrtc/modules/remote_bitrate_estimator/test/packet_sender.h
@@ -109,7 +109,6 @@ class TcpSender : public PacketSender {
         now_ms_(0),
         in_slow_start_(false),
         cwnd_(10),
-        in_flight_(0),
         ack_received_(false),
         last_acked_seq_num_(0),
         next_sequence_number_(0) {}
@@ -120,6 +119,25 @@ class TcpSender : public PacketSender {
   int GetFeedbackIntervalMs() const override { return 10; }
 
  private:
+  struct InFlight {
+   public:
+    InFlight(const MediaPacket& packet)
+        : sequence_number(packet.header().sequenceNumber),
+          time_ms(packet.send_time_us() / 1000) {}
+
+    InFlight(uint16_t seq_num, int64_t now_ms)
+        : sequence_number(seq_num), time_ms(now_ms) {}
+
+    bool operator<(const InFlight& rhs) const {
+      return sequence_number < rhs.sequence_number;
+    }
+
+    uint16_t sequence_number;  // Sequence number of a packet in flight, or a
+                               // packet which has just been acked.
+    int64_t time_ms;  // Time of when the packet left the sender, or when the
+                      // ack was received.
+  };
+
   void SendPackets(Packets* in_out);
   void UpdateCongestionControl(const FeedbackPacket* fb);
   Packets GeneratePackets(size_t num_packets);
@@ -127,7 +145,7 @@ class TcpSender : public PacketSender {
   int64_t now_ms_;
   bool in_slow_start_;
   float cwnd_;
-  int in_flight_;
+  std::set<InFlight> in_flight_;
   bool ack_received_;
   uint16_t last_acked_seq_num_;
   uint16_t next_sequence_number_;