diff --git a/api/transport/network_types.h b/api/transport/network_types.h
index 0693f69c7a..155d4ec11e 100644
--- a/api/transport/network_types.h
+++ b/api/transport/network_types.h
@@ -89,6 +89,8 @@ struct SentPacket {
   // sequence number that is unique over the whole call and increasing by 1 for
   // each packet.
   int64_t sequence_number;
+  // Data in flight when the packet was sent, including the packet.
+  DataSize data_in_flight = DataSize::Zero();
 };
 
 // Transport level feedback
diff --git a/api/transport/test/network_control_tester.cc b/api/transport/test/network_control_tester.cc
index 7ce75b17ed..0ba53028ef 100644
--- a/api/transport/test/network_control_tester.cc
+++ b/api/transport/test/network_control_tester.cc
@@ -95,6 +95,9 @@ void NetworkControllerTester::RunSimulation(TimeDelta duration,
       SentPacket sent_packet;
       sent_packet = next_packet(state_, current_time_, packet_interval);
       sent_packet.sequence_number = packet_sequence_number_++;
+      sent_packet.data_in_flight = sent_packet.size;
+      for (PacketResult& packet : outstanding_packets_)
+        sent_packet.data_in_flight += packet.sent_packet->size;
       Update(&state_, controller_->OnSentPacket(sent_packet));
 
       accumulated_buffer_ += sent_packet.size;
diff --git a/modules/congestion_controller/rtp/send_side_congestion_controller.cc b/modules/congestion_controller/rtp/send_side_congestion_controller.cc
index 4ee5b7c4f8..de2fa9af59 100644
--- a/modules/congestion_controller/rtp/send_side_congestion_controller.cc
+++ b/modules/congestion_controller/rtp/send_side_congestion_controller.cc
@@ -594,6 +594,8 @@ void SendSideCongestionController::OnSentPacket(
     msg.size = DataSize::bytes(packet->payload_size);
     msg.send_time = Timestamp::ms(packet->send_time_ms);
     msg.sequence_number = packet->long_sequence_number;
+    msg.data_in_flight =
+        DataSize::bytes(transport_feedback_adapter_.GetOutstandingBytes());
     task_queue_->PostTask([this, msg]() {
       RTC_DCHECK_RUN_ON(task_queue_);
       if (controller_)