Back off relative to current estimate rather than ack rate when in ALR.

If we're in ALR, the acked rate is going to be significantly lower than
the current estimate for the link capacity. If we need to back off in
this situation (usually caused by latency spikes), this CL makes us back
off relative to current estimate if. We then immediately send a new
probe just in case the network did actually change.

All of this is behind experiment flags for now.

Bug: webrtc:10144
Change-Id: I062a259c36417eea2211d44592ef7fc979aa22b7
Reviewed-on: https://webrtc-review.googlesource.com/c/113880
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26045}

modules/congestion_controller/goog_cc/delay_based_bwe.cc

@@ -70,13 +70,15 @@ DelayBasedBwe::Result::Result()
     : updated(false),
       probe(false),
       target_bitrate(DataRate::Zero()),
-      recovered_from_overuse(false) {}
+      recovered_from_overuse(false),
+      backoff_in_alr(false) {}
 
 DelayBasedBwe::Result::Result(bool probe, DataRate target_bitrate)
     : updated(true),
       probe(probe),
       target_bitrate(target_bitrate),
-      recovered_from_overuse(false) {}
+      recovered_from_overuse(false),
+      backoff_in_alr(false) {}
 
 DelayBasedBwe::Result::~Result() {}
 
@@ -93,7 +95,9 @@ DelayBasedBwe::DelayBasedBwe(RtcEventLog* event_log)
       trendline_smoothing_coeff_(kDefaultTrendlineSmoothingCoeff),
       trendline_threshold_gain_(kDefaultTrendlineThresholdGain),
       prev_bitrate_(DataRate::Zero()),
-      prev_state_(BandwidthUsage::kBwNormal) {
+      prev_state_(BandwidthUsage::kBwNormal),
+      alr_limited_backoff_enabled_(
+          field_trial::IsEnabled("WebRTC-Bwe-AlrLimitedBackoff")) {
   RTC_LOG(LS_INFO)
       << "Using Trendline filter for delay change estimation with window size "
       << trendline_window_size_;
@@ -108,6 +112,7 @@ DelayBasedBwe::Result DelayBasedBwe::IncomingPacketFeedbackVector(
     const std::vector<PacketFeedback>& packet_feedback_vector,
     absl::optional<DataRate> acked_bitrate,
     absl::optional<DataRate> probe_bitrate,
+    bool in_alr,
     Timestamp at_time) {
   RTC_DCHECK(std::is_sorted(packet_feedback_vector.begin(),
                             packet_feedback_vector.end(),
@@ -149,7 +154,7 @@ DelayBasedBwe::Result DelayBasedBwe::IncomingPacketFeedbackVector(
     return Result();
   }
   return MaybeUpdateEstimate(acked_bitrate, probe_bitrate,
-                             recovered_from_overuse, at_time);
+                             recovered_from_overuse, in_alr, at_time);
 }
 
 void DelayBasedBwe::IncomingPacketFeedback(
@@ -194,13 +199,19 @@ DelayBasedBwe::Result DelayBasedBwe::MaybeUpdateEstimate(
     absl::optional<DataRate> acked_bitrate,
     absl::optional<DataRate> probe_bitrate,
     bool recovered_from_overuse,
+    bool in_alr,
     Timestamp at_time) {
   Result result;
 
   // Currently overusing the bandwidth.
   if (delay_detector_->State() == BandwidthUsage::kBwOverusing) {
-    if (acked_bitrate &&
+    if (in_alr && alr_limited_backoff_enabled_ &&
-        rate_control_.TimeToReduceFurther(at_time, *acked_bitrate)) {
+        rate_control_.TimeToReduceFurther(at_time, prev_bitrate_)) {
+      result.updated =
+          UpdateEstimate(at_time, prev_bitrate_, &result.target_bitrate);
+      result.backoff_in_alr = true;
+    } else if (acked_bitrate &&
+               rate_control_.TimeToReduceFurther(at_time, *acked_bitrate)) {
       result.updated =
           UpdateEstimate(at_time, acked_bitrate, &result.target_bitrate);
     } else if (!acked_bitrate && rate_control_.ValidEstimate() &&
@@ -287,4 +298,9 @@ void DelayBasedBwe::SetMinBitrate(DataRate min_bitrate) {
 TimeDelta DelayBasedBwe::GetExpectedBwePeriod() const {
   return rate_control_.GetExpectedBandwidthPeriod();
 }
+
+void DelayBasedBwe::SetAlrLimitedBackoffExperiment(bool enabled) {
+  alr_limited_backoff_enabled_ = enabled;
+}
+
 }  // namespace webrtc

modules/congestion_controller/goog_cc/delay_based_bwe.h

@@ -39,6 +39,7 @@ class DelayBasedBwe {
     bool probe;
     DataRate target_bitrate = DataRate::Zero();
     bool recovered_from_overuse;
+    bool backoff_in_alr;
   };
 
   explicit DelayBasedBwe(RtcEventLog* event_log);
@@ -48,12 +49,14 @@ class DelayBasedBwe {
       const std::vector<PacketFeedback>& packet_feedback_vector,
       absl::optional<DataRate> acked_bitrate,
       absl::optional<DataRate> probe_bitrate,
+      bool in_alr,
       Timestamp at_time);
   void OnRttUpdate(TimeDelta avg_rtt);
   bool LatestEstimate(std::vector<uint32_t>* ssrcs, DataRate* bitrate) const;
   void SetStartBitrate(DataRate start_bitrate);
   void SetMinBitrate(DataRate min_bitrate);
   TimeDelta GetExpectedBwePeriod() const;
+  void SetAlrLimitedBackoffExperiment(bool enabled);
 
  private:
   friend class GoogCcStatePrinter;
@@ -61,7 +64,8 @@ class DelayBasedBwe {
                               Timestamp at_time);
   Result MaybeUpdateEstimate(absl::optional<DataRate> acked_bitrate,
                              absl::optional<DataRate> probe_bitrate,
-                             bool request_probe,
+                             bool recovered_from_overuse,
+                             bool in_alr,
                              Timestamp at_time);
   // Updates the current remote rate estimate and returns true if a valid
   // estimate exists.
@@ -81,6 +85,7 @@ class DelayBasedBwe {
   double trendline_threshold_gain_;
   DataRate prev_bitrate_;
   BandwidthUsage prev_state_;
+  bool alr_limited_backoff_enabled_;
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(DelayBasedBwe);
 };

modules/congestion_controller/goog_cc/delay_based_bwe_unittest.cc

@@ -29,8 +29,9 @@ constexpr Timestamp kDummyTimestamp = Timestamp::Seconds<1000>();
 
 TEST_F(DelayBasedBweTest, NoCrashEmptyFeedback) {
   std::vector<PacketFeedback> packet_feedback_vector;
-  bitrate_estimator_->IncomingPacketFeedbackVector(
+  bitrate_estimator_->IncomingPacketFeedbackVector(packet_feedback_vector,
-      packet_feedback_vector, absl::nullopt, absl::nullopt, kDummyTimestamp);
+                                                   absl::nullopt, absl::nullopt,
+                                                   false, kDummyTimestamp);
 }
 
 TEST_F(DelayBasedBweTest, NoCrashOnlyLostFeedback) {
@@ -41,8 +42,9 @@ TEST_F(DelayBasedBweTest, NoCrashOnlyLostFeedback) {
   packet_feedback_vector.push_back(PacketFeedback(PacketFeedback::kNotReceived,
                                                   PacketFeedback::kNoSendTime,
                                                   1, 1500, PacedPacketInfo()));
-  bitrate_estimator_->IncomingPacketFeedbackVector(
+  bitrate_estimator_->IncomingPacketFeedbackVector(packet_feedback_vector,
-      packet_feedback_vector, absl::nullopt, absl::nullopt, kDummyTimestamp);
+                                                   absl::nullopt, absl::nullopt,
+                                                   false, kDummyTimestamp);
 }
 
 TEST_F(DelayBasedBweTest, ProbeDetection) {

modules/congestion_controller/goog_cc/delay_based_bwe_unittest_helper.cc

@@ -207,7 +207,7 @@ void DelayBasedBweTest::IncomingFeedback(int64_t arrival_time_ms,
   DelayBasedBwe::Result result =
       bitrate_estimator_->IncomingPacketFeedbackVector(
           packets, acknowledged_bitrate_estimator_->bitrate(),
-          probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate(),
+          probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate(), false,
           Timestamp::ms(clock_.TimeInMilliseconds()));
   const uint32_t kDummySsrc = 0;
   if (result.updated) {
@@ -248,7 +248,7 @@ bool DelayBasedBweTest::GenerateAndProcessFrame(uint32_t ssrc,
   DelayBasedBwe::Result result =
       bitrate_estimator_->IncomingPacketFeedbackVector(
           packets, acknowledged_bitrate_estimator_->bitrate(),
-          probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate(),
+          probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate(), false,
           Timestamp::ms(clock_.TimeInMilliseconds()));
   const uint32_t kDummySsrc = 0;
   if (result.updated) {

modules/congestion_controller/goog_cc/goog_cc_network_control.cc

@@ -498,7 +498,7 @@ NetworkControlUpdate GoogCcNetworkController::OnTransportPacketsFeedback(
   DelayBasedBwe::Result result;
   result = delay_based_bwe_->IncomingPacketFeedbackVector(
       received_feedback_vector, acknowledged_bitrate, probe_bitrate,
-      report.feedback_time);
+      alr_start_time.has_value(), report.feedback_time);
 
   NetworkControlUpdate update;
   if (result.updated) {
@@ -518,6 +518,11 @@ NetworkControlUpdate GoogCcNetworkController::OnTransportPacketsFeedback(
     auto probes = probe_controller_->RequestProbe(report.feedback_time.ms());
     update.probe_cluster_configs.insert(update.probe_cluster_configs.end(),
                                         probes.begin(), probes.end());
+  } else if (result.backoff_in_alr) {
+    // If we just backed off during ALR, request a new probe.
+    auto probes = probe_controller_->RequestProbe(report.feedback_time.ms());
+    update.probe_cluster_configs.insert(update.probe_cluster_configs.end(),
+                                        probes.begin(), probes.end());
   }
 
   // No valid RTT could be because send-side BWE isn't used, in which case

modules/congestion_controller/include/send_side_congestion_controller.h

@@ -119,6 +119,8 @@ class DEPRECATED_SendSideCongestionController
   void EnableCongestionWindowPushback(int64_t accepted_queue_ms,
                                       uint32_t min_pushback_target_bitrate_bps);
 
+  void SetAlrLimitedBackoffExperiment(bool enable);
+
  private:
   void MaybeTriggerOnNetworkChanged();
 

modules/congestion_controller/send_side_congestion_controller.cc

@@ -186,6 +186,12 @@ void DEPRECATED_SendSideCongestionController::EnableCongestionWindowPushback(
           min_pushback_target_bitrate_bps);
 }
 
+void DEPRECATED_SendSideCongestionController::SetAlrLimitedBackoffExperiment(
+    bool enable) {
+  rtc::CritScope cs(&bwe_lock_);
+  delay_based_bwe_->SetAlrLimitedBackoffExperiment(enable);
+}
+
 void DEPRECATED_SendSideCongestionController::RegisterPacketFeedbackObserver(
     PacketFeedbackObserver* observer) {
   transport_feedback_adapter_.RegisterPacketFeedbackObserver(observer);
@@ -437,7 +443,7 @@ void DEPRECATED_SendSideCongestionController::OnTransportFeedback(
     result = delay_based_bwe_->IncomingPacketFeedbackVector(
         feedback_vector, acknowledged_bitrate_estimator_->bitrate(),
         probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate(),
-        Timestamp::ms(clock_->TimeInMilliseconds()));
+        currently_in_alr, Timestamp::ms(clock_->TimeInMilliseconds()));
   }
   if (result.updated) {
     bitrate_controller_->OnDelayBasedBweResult(result);
@@ -449,6 +455,9 @@ void DEPRECATED_SendSideCongestionController::OnTransportFeedback(
     probe_controller_->SetAlrStartTimeMs(
         pacer_->GetApplicationLimitedRegionStartTime());
     SendProbes(probe_controller_->RequestProbe(clock_->TimeInMilliseconds()));
+  } else if (result.backoff_in_alr) {
+    rtc::CritScope cs(&probe_lock_);
+    SendProbes(probe_controller_->RequestProbe(clock_->TimeInMilliseconds()));
   }
   if (in_cwnd_experiment_) {
     LimitOutstandingBytes(transport_feedback_adapter_.GetOutstandingBytes());

modules/remote_bitrate_estimator/test/estimators/send_side.cc

@@ -91,7 +91,7 @@ void SendSideBweSender::GiveFeedback(const FeedbackPacket& feedback) {
   }
   DelayBasedBwe::Result result = bwe_->IncomingPacketFeedbackVector(
       packet_feedback_vector, acknowledged_bitrate_estimator_->bitrate(),
-      probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate(),
+      probe_bitrate_estimator_->FetchAndResetLastEstimatedBitrate(), false,
       Timestamp::ms(clock_->TimeInMilliseconds()));
   if (result.updated)
     bitrate_controller_->OnDelayBasedBweResult(result);