diff --git a/api/transport/network_types.h b/api/transport/network_types.h
index 0f1d7ab683..43778caaac 100644
--- a/api/transport/network_types.h
+++ b/api/transport/network_types.h
@@ -136,6 +136,7 @@ struct TransportPacketsFeedback {
   ~TransportPacketsFeedback();
 
   Timestamp feedback_time = Timestamp::PlusInfinity();
+  Timestamp first_unacked_send_time = Timestamp::PlusInfinity();
   DataSize data_in_flight = DataSize::Zero();
   DataSize prior_in_flight = DataSize::Zero();
   std::vector<PacketResult> packet_feedbacks;
diff --git a/modules/congestion_controller/rtp/send_time_history.cc b/modules/congestion_controller/rtp/send_time_history.cc
index 110dac3993..5011da1d1a 100644
--- a/modules/congestion_controller/rtp/send_time_history.cc
+++ b/modules/congestion_controller/rtp/send_time_history.cc
@@ -121,6 +121,16 @@ DataSize SendTimeHistory::GetOutstandingData(uint16_t local_net_id,
   }
 }
 
+absl::optional<int64_t> SendTimeHistory::GetFirstUnackedSendTime() const {
+  if (!last_ack_seq_num_)
+    return absl::nullopt;
+  auto it = history_.find(*last_ack_seq_num_);
+  if (it == history_.end() ||
+      it->second.send_time_ms == PacketFeedback::kNoSendTime)
+    return absl::nullopt;
+  return it->second.send_time_ms;
+}
+
 void SendTimeHistory::AddPacketBytes(const PacketFeedback& packet) {
   if (packet.send_time_ms < 0 || packet.payload_size == 0 ||
       (last_ack_seq_num_ && *last_ack_seq_num_ >= packet.long_sequence_number))
diff --git a/modules/congestion_controller/rtp/send_time_history.h b/modules/congestion_controller/rtp/send_time_history.h
index 656b7e16a3..c28ca61507 100644
--- a/modules/congestion_controller/rtp/send_time_history.h
+++ b/modules/congestion_controller/rtp/send_time_history.h
@@ -47,6 +47,8 @@ class SendTimeHistory {
   DataSize GetOutstandingData(uint16_t local_net_id,
                               uint16_t remote_net_id) const;
 
+  absl::optional<int64_t> GetFirstUnackedSendTime() const;
+
  private:
   using RemoteAndLocalNetworkId = std::pair<uint16_t, uint16_t>;
 
diff --git a/modules/congestion_controller/rtp/transport_feedback_adapter.cc b/modules/congestion_controller/rtp/transport_feedback_adapter.cc
index c4bbc1921a..662cbef7ea 100644
--- a/modules/congestion_controller/rtp/transport_feedback_adapter.cc
+++ b/modules/congestion_controller/rtp/transport_feedback_adapter.cc
@@ -148,6 +148,13 @@ TransportFeedbackAdapter::ProcessTransportFeedback(
           Timestamp::ms(rtp_feedback.arrival_time_ms));
     }
   }
+  {
+    rtc::CritScope cs(&lock_);
+    absl::optional<int64_t> first_unacked_send_time_ms =
+        send_time_history_.GetFirstUnackedSendTime();
+    if (first_unacked_send_time_ms)
+      msg.first_unacked_send_time = Timestamp::ms(*first_unacked_send_time_ms);
+  }
   msg.feedback_time = Timestamp::ms(feedback_time_ms);
   msg.prior_in_flight = prior_in_flight;
   msg.data_in_flight = GetOutstandingData();