Implement periodic bandwidth probing in application-limited region.

Now ProbeController can send periodic bandwidth probes when in
application-limited region. This will allow to maintain correct
bottleneck bandwidth estimate, even not all bandwidth is being used.
The feature is not enabled by default, but can be enabled with a flag.
Interval between probes is currently set to 5 seconds.

BUG=webrtc:6332

Review-Url: https://codereview.webrtc.org/2504023002
Cr-Commit-Position: refs/heads/master@{#15279}

webrtc/media/base/mediachannel.h

@@ -129,6 +129,9 @@ struct MediaConfig {
     // VideoReceiveStream, where the value is passed on to the
     // IncomingVideoStream constructor.
     bool disable_prerenderer_smoothing = false;
+
+    // Enables periodic bandwidth probing in application-limited region.
+    bool periodic_alr_bandwidth_probing = false;
   } video;
 };
 

webrtc/media/engine/webrtcvideoengine2.cc

@@ -1102,6 +1102,8 @@ bool WebRtcVideoChannel2::AddSendStream(const StreamParams& sp) {
 
   webrtc::VideoSendStream::Config config(this);
   config.suspend_below_min_bitrate = video_config_.suspend_below_min_bitrate;
+  config.periodic_alr_bandwidth_probing =
+      video_config_.periodic_alr_bandwidth_probing;
   WebRtcVideoSendStream* stream = new WebRtcVideoSendStream(
       call_, sp, std::move(config), default_send_options_,
       external_encoder_factory_, video_config_.enable_cpu_overuse_detection,

webrtc/modules/congestion_controller/congestion_controller.cc

@@ -275,6 +275,10 @@ RateLimiter* CongestionController::GetRetransmissionRateLimiter() {
   return retransmission_rate_limiter_.get();
 }
 
+void CongestionController::EnablePeriodicAlrProbing(bool enable) {
+  probe_controller_->EnablePeriodicAlrProbing(enable);
+}
+
 void CongestionController::SetAllocatedSendBitrateLimits(
     int min_send_bitrate_bps,
     int max_padding_bitrate_bps) {
@@ -319,6 +323,7 @@ int64_t CongestionController::TimeUntilNextProcess() {
 void CongestionController::Process() {
   bitrate_controller_->Process();
   remote_bitrate_estimator_->Process();
+  probe_controller_->Process();
   MaybeTriggerOnNetworkChanged();
 }
 

webrtc/modules/congestion_controller/include/congestion_controller.h

@@ -82,6 +82,7 @@ class CongestionController : public CallStatsObserver, public Module {
   virtual PacketRouter* packet_router() { return packet_router_.get(); }
   virtual TransportFeedbackObserver* GetTransportFeedbackObserver();
   RateLimiter* GetRetransmissionRateLimiter();
+  void EnablePeriodicAlrProbing(bool enable);
 
   // SetAllocatedSendBitrateLimits sets bitrates limits imposed by send codec
   // settings.

webrtc/modules/congestion_controller/probe_controller.cc

@@ -38,8 +38,14 @@ constexpr int kExponentialProbingDisabled = 0;
 constexpr int kDefaultMaxProbingBitrateBps = 100000000;
 
 // This is a limit on how often probing can be done when there is a BW
-// drop detected in ALR region.
-constexpr int kAlrProbingIntervalLimitMs = 5000;
+// drop detected in ALR.
+constexpr int64_t kAlrProbingIntervalMinMs = 5000;
+
+// Interval between probes when ALR periodic probing is enabled.
+constexpr int64_t kAlrPeriodicProbingIntervalMs = 5000;
+
+// Minimum probe bitrate percentage to probe further for repeated probes.
+constexpr int kRepeatedProbeMinPercentage = 125;
 
 }  // namespace
 
@@ -53,7 +59,8 @@ ProbeController::ProbeController(PacedSender* pacer, Clock* clock)
       estimated_bitrate_bps_(0),
       start_bitrate_bps_(0),
       max_bitrate_bps_(0),
-      last_alr_probing_time_(clock_->TimeInMilliseconds()) {}
+      last_alr_probing_time_(clock_->TimeInMilliseconds()),
+      enable_periodic_alr_probing_(false) {}
 
 void ProbeController::SetBitrates(int min_bitrate_bps,
                                   int start_bitrate_bps,
@@ -83,7 +90,8 @@ void ProbeController::SetBitrates(int min_bitrate_bps,
       if (estimated_bitrate_bps_ != 0 &&
           estimated_bitrate_bps_ < old_max_bitrate_bps &&
           max_bitrate_bps_ > old_max_bitrate_bps) {
-        InitiateProbing({max_bitrate_bps}, kExponentialProbingDisabled);
+        InitiateProbing(clock_->TimeInMilliseconds(), {max_bitrate_bps},
+                        kExponentialProbingDisabled);
       }
       break;
   }
@@ -103,14 +111,17 @@ void ProbeController::InitiateExponentialProbing() {
 
   // When probing at 1.8 Mbps ( 6x 300), this represents a threshold of
   // 1.2 Mbps to continue probing.
-  InitiateProbing({3 * start_bitrate_bps_, 6 * start_bitrate_bps_},
+  InitiateProbing(clock_->TimeInMilliseconds(),
+                  {3 * start_bitrate_bps_, 6 * start_bitrate_bps_},
                   4 * start_bitrate_bps_);
 }
 
 void ProbeController::SetEstimatedBitrate(int bitrate_bps) {
   rtc::CritScope cs(&critsect_);
+  int64_t now_ms = clock_->TimeInMilliseconds();
+
   if (state_ == State::kWaitingForProbingResult) {
-    if ((clock_->TimeInMilliseconds() - time_last_probing_initiated_ms_) >
+    if ((now_ms - time_last_probing_initiated_ms_) >
         kMaxWaitingTimeForProbingResultMs) {
       LOG(LS_INFO) << "kWaitingForProbingResult: timeout";
       state_ = State::kProbingComplete;
@@ -125,38 +136,73 @@ void ProbeController::SetEstimatedBitrate(int bitrate_bps) {
           bitrate_bps > min_bitrate_to_probe_further_bps_) {
         // Double the probing bitrate and expect a minimum of 25% gain to
         // continue probing.
-        InitiateProbing({2 * bitrate_bps}, 1.25 * bitrate_bps);
-      }
-    }
-  } else {
-    // A drop in estimated BW when operating in ALR and not already probing.
-    // The current response is to initiate a single probe session at the
-    // previous bitrate and immediately use the reported bitrate as the new
-    // bitrate.
-    //
-    // If the probe session fails, the assumption is that this drop was a
-    // real one from a competing flow or something else on the network and
-    // it ramps up from bitrate_bps.
-    if (pacer_->InApplicationLimitedRegion() &&
-        bitrate_bps < 0.5 * estimated_bitrate_bps_) {
-      int64_t now_ms = clock_->TimeInMilliseconds();
-      if ((now_ms - last_alr_probing_time_) > kAlrProbingIntervalLimitMs) {
-        LOG(LS_INFO) << "Detected big BW drop in ALR, start probe.";
-        // Track how often we probe in response to BW drop in ALR.
-        RTC_HISTOGRAM_COUNTS_10000("WebRTC.BWE.AlrProbingIntervalInS",
-                                   (now_ms - last_alr_probing_time_) / 1000);
-        InitiateProbing({estimated_bitrate_bps_}, kExponentialProbingDisabled);
-        last_alr_probing_time_ = now_ms;
+        InitiateProbing(now_ms, {2 * bitrate_bps},
+                        bitrate_bps * kRepeatedProbeMinPercentage / 100);
+      } else {
+        // Stop exponential probing.
+        state_ = State::kProbingComplete;
+        min_bitrate_to_probe_further_bps_ = kExponentialProbingDisabled;
       }
     }
+  }
+
+  // Detect a drop in estimated BW when operating in ALR and not already
+  // probing. The current response is to initiate a single probe session at the
+  // previous bitrate and immediately use the reported bitrate as the new
+  // bitrate.
+  //
+  // If the probe session fails, the assumption is that this drop was a
+  // real one from a competing flow or something else on the network and
+  // it ramps up from bitrate_bps.
+  if (state_ == State::kProbingComplete &&
+      pacer_->GetApplicationLimitedRegionStartTime() &&
+      bitrate_bps < estimated_bitrate_bps_ / 2 &&
+      (now_ms - last_alr_probing_time_) > kAlrProbingIntervalMinMs) {
+    LOG(LS_INFO) << "Detected big BW drop in ALR, start probe.";
+    // Track how often we probe in response to BW drop in ALR.
+    RTC_HISTOGRAM_COUNTS_10000("WebRTC.BWE.AlrProbingIntervalInS",
+                               (now_ms - last_alr_probing_time_) / 1000);
+    InitiateProbing(now_ms, {estimated_bitrate_bps_},
+                    kExponentialProbingDisabled);
+    last_alr_probing_time_ = now_ms;
+
     // TODO(isheriff): May want to track when we did ALR probing in order
     // to reset |last_alr_probing_time_| if we validate that it was a
     // drop due to exogenous event.
   }
+
   estimated_bitrate_bps_ = bitrate_bps;
 }
 
+void ProbeController::EnablePeriodicAlrProbing(bool enable) {
+  rtc::CritScope cs(&critsect_);
+  enable_periodic_alr_probing_ = enable;
+}
+
+void ProbeController::Process() {
+  rtc::CritScope cs(&critsect_);
+
+  if (state_ != State::kProbingComplete || !enable_periodic_alr_probing_)
+    return;
+
+  // Probe bandwidth periodically when in ALR state.
+  rtc::Optional<int64_t> alr_start_time =
+      pacer_->GetApplicationLimitedRegionStartTime();
+  if (alr_start_time) {
+    int64_t now_ms = clock_->TimeInMilliseconds();
+    int64_t next_probe_time_ms =
+        std::max(*alr_start_time, time_last_probing_initiated_ms_) +
+        kAlrPeriodicProbingIntervalMs;
+    if (now_ms >= next_probe_time_ms) {
+      InitiateProbing(
+          now_ms, {estimated_bitrate_bps_ * 2},
+          estimated_bitrate_bps_ * kRepeatedProbeMinPercentage / 100);
+    }
+  }
+}
+
 void ProbeController::InitiateProbing(
+    int64_t now_ms,
     std::initializer_list<int> bitrates_to_probe,
     int min_bitrate_to_probe_further_bps) {
   bool first_cluster = true;
@@ -172,7 +218,7 @@ void ProbeController::InitiateProbing(
     }
   }
   min_bitrate_to_probe_further_bps_ = min_bitrate_to_probe_further_bps;
-  time_last_probing_initiated_ms_ = clock_->TimeInMilliseconds();
+  time_last_probing_initiated_ms_ = now_ms;
   if (min_bitrate_to_probe_further_bps == kExponentialProbingDisabled)
     state_ = State::kProbingComplete;
   else

webrtc/modules/congestion_controller/probe_controller.h

@@ -36,6 +36,9 @@ class ProbeController {
 
   void SetEstimatedBitrate(int bitrate_bps);
 
+  void EnablePeriodicAlrProbing(bool enable);
+  void Process();
+
  private:
   enum class State {
     // Initial state where no probing has been triggered yet.
@@ -47,7 +50,8 @@ class ProbeController {
   };
 
   void InitiateExponentialProbing() EXCLUSIVE_LOCKS_REQUIRED(critsect_);
-  void InitiateProbing(std::initializer_list<int> bitrates_to_probe,
+  void InitiateProbing(int64_t now_ms,
+                       std::initializer_list<int> bitrates_to_probe,
                        int min_bitrate_to_probe_further_bps)
       EXCLUSIVE_LOCKS_REQUIRED(critsect_);
 
@@ -62,6 +66,7 @@ class ProbeController {
   int start_bitrate_bps_ GUARDED_BY(critsect_);
   int max_bitrate_bps_ GUARDED_BY(critsect_);
   int64_t last_alr_probing_time_ GUARDED_BY(critsect_);
+  bool enable_periodic_alr_probing_ GUARDED_BY(critsect_);
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(ProbeController);
 };

webrtc/modules/congestion_controller/probe_controller_unittest.cc

@@ -19,6 +19,7 @@
 using testing::_;
 using testing::AtLeast;
 using testing::NiceMock;
+using testing::Return;
 
 namespace webrtc {
 namespace test {
@@ -31,11 +32,13 @@ constexpr int kMaxBitrateBps = 10000;
 
 constexpr int kExponentialProbingTimeoutMs = 5000;
 
+constexpr int kAlrProbeInterval = 5000;
+
 }  // namespace
 
 class ProbeControllerTest : public ::testing::Test {
  protected:
-  ProbeControllerTest() : clock_(0) {
+  ProbeControllerTest() : clock_(100000000L) {
     probe_controller_.reset(new ProbeController(&pacer_, &clock_));
   }
   ~ProbeControllerTest() override {}
@@ -79,8 +82,6 @@ TEST_F(ProbeControllerTest, TestExponentialProbing) {
   probe_controller_->SetBitrates(kMinBitrateBps, kStartBitrateBps,
                                  kMaxBitrateBps);
 
-
-
   EXPECT_CALL(pacer_, CreateProbeCluster(2 * 1800, _));
   probe_controller_->SetEstimatedBitrate(1800);
 }
@@ -95,5 +96,59 @@ TEST_F(ProbeControllerTest, TestExponentialProbingTimeout) {
   probe_controller_->SetEstimatedBitrate(1800);
 }
 
+TEST_F(ProbeControllerTest, ProbeAfterEstimateDropInAlr) {
+  EXPECT_CALL(pacer_, CreateProbeCluster(_, _)).Times(2);
+  probe_controller_->SetBitrates(kMinBitrateBps, kStartBitrateBps,
+                                 kMaxBitrateBps);
+  probe_controller_->SetEstimatedBitrate(500);
+  testing::Mock::VerifyAndClearExpectations(&pacer_);
+
+  // When bandwidth estimate drops the controller should send a probe at the
+  // previous bitrate.
+  EXPECT_CALL(pacer_, CreateProbeCluster(500, _)).Times(1);
+  EXPECT_CALL(pacer_, GetApplicationLimitedRegionStartTime())
+      .WillRepeatedly(
+          Return(rtc::Optional<int64_t>(clock_.TimeInMilliseconds())));
+  clock_.AdvanceTimeMilliseconds(kAlrProbeInterval + 1);
+  probe_controller_->SetEstimatedBitrate(50);
+}
+
+TEST_F(ProbeControllerTest, PeriodicProbing) {
+  EXPECT_CALL(pacer_, CreateProbeCluster(_, _)).Times(2);
+  probe_controller_->EnablePeriodicAlrProbing(true);
+  probe_controller_->SetBitrates(kMinBitrateBps, kStartBitrateBps,
+                                 kMaxBitrateBps);
+  probe_controller_->SetEstimatedBitrate(500);
+  testing::Mock::VerifyAndClearExpectations(&pacer_);
+
+  int64_t start_time = clock_.TimeInMilliseconds();
+
+  // Expect the controller to send a new probe after 5s has passed.
+  EXPECT_CALL(pacer_, CreateProbeCluster(1000, _)).Times(1);
+  EXPECT_CALL(pacer_, GetApplicationLimitedRegionStartTime())
+      .WillRepeatedly(Return(rtc::Optional<int64_t>(start_time)));
+  clock_.AdvanceTimeMilliseconds(5000);
+  probe_controller_->Process();
+  probe_controller_->SetEstimatedBitrate(500);
+  testing::Mock::VerifyAndClearExpectations(&pacer_);
+
+  // The following probe should be sent at 10s into ALR.
+  EXPECT_CALL(pacer_, CreateProbeCluster(_, _)).Times(0);
+  EXPECT_CALL(pacer_, GetApplicationLimitedRegionStartTime())
+      .WillRepeatedly(Return(rtc::Optional<int64_t>(start_time)));
+  clock_.AdvanceTimeMilliseconds(4000);
+  probe_controller_->Process();
+  probe_controller_->SetEstimatedBitrate(500);
+  testing::Mock::VerifyAndClearExpectations(&pacer_);
+
+  EXPECT_CALL(pacer_, CreateProbeCluster(_, _)).Times(1);
+  EXPECT_CALL(pacer_, GetApplicationLimitedRegionStartTime())
+      .WillRepeatedly(Return(rtc::Optional<int64_t>(start_time)));
+  clock_.AdvanceTimeMilliseconds(1000);
+  probe_controller_->Process();
+  probe_controller_->SetEstimatedBitrate(500);
+  testing::Mock::VerifyAndClearExpectations(&pacer_);
+}
+
 }  // namespace test
 }  // namespace webrtc

webrtc/modules/pacing/alr_detector.cc

@@ -44,10 +44,10 @@ void AlrDetector::OnBytesSent(size_t bytes_sent, int64_t now_ms) {
     return;
 
   int percentage = static_cast<int>(*rate) * 100 / estimated_bitrate_bps_;
-  if (percentage < kAlrStartUsagePercent && !application_limited_) {
-    application_limited_ = true;
-  } else if (percentage > kAlrEndUsagePercent && application_limited_) {
-    application_limited_ = false;
+  if (percentage < kAlrStartUsagePercent && !alr_started_time_ms_) {
+    alr_started_time_ms_ = rtc::Optional<int64_t>(now_ms);
+  } else if (percentage > kAlrEndUsagePercent && alr_started_time_ms_) {
+    alr_started_time_ms_ = rtc::Optional<int64_t>();
   }
 }
 
@@ -56,8 +56,9 @@ void AlrDetector::SetEstimatedBitrate(int bitrate_bps) {
   estimated_bitrate_bps_ = bitrate_bps;
 }
 
-bool AlrDetector::InApplicationLimitedRegion() const {
-  return application_limited_;
+rtc::Optional<int64_t> AlrDetector::GetApplicationLimitedRegionStartTime()
+    const {
+  return alr_started_time_ms_;
 }
 
 }  // namespace webrtc

webrtc/modules/pacing/alr_detector.h

@@ -35,13 +35,16 @@ class AlrDetector {
   // Set current estimated bandwidth.
   void SetEstimatedBitrate(int bitrate_bps);
 
-  // Returns true if currently in application-limited region.
-  bool InApplicationLimitedRegion() const;
+  // Returns time in milliseconds when the current application-limited region
+  // started or empty result if the sender is currently not application-limited.
+  rtc::Optional<int64_t> GetApplicationLimitedRegionStartTime() const;
 
  private:
   RateStatistics rate_;
   int estimated_bitrate_bps_ = 0;
-  bool application_limited_ = false;
+
+  // Non-empty in ALR state.
+  rtc::Optional<int64_t> alr_started_time_ms_;
 };
 
 }  // namespace webrtc

webrtc/modules/pacing/alr_detector_unittest.cc

@@ -51,61 +51,61 @@ class AlrDetectorTest : public testing::Test {
 
 TEST_F(AlrDetectorTest, AlrDetection) {
   // Start in non-ALR state.
-  EXPECT_FALSE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_FALSE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // Stay in non-ALR state when usage is close to 100%.
   SimulateOutgoingTraffic(500, 90);
-  EXPECT_FALSE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_FALSE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // Verify that we ALR starts when bitrate drops below 20%.
   SimulateOutgoingTraffic(500, 20);
-  EXPECT_TRUE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_TRUE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // Verify that we remain in ALR state while usage is still below 50%.
   SimulateOutgoingTraffic(500, 40);
-  EXPECT_TRUE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_TRUE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // Verify that ALR ends when usage is above 50%.
   SimulateOutgoingTraffic(500, 60);
-  EXPECT_FALSE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_FALSE(alr_detector_.GetApplicationLimitedRegionStartTime());
 }
 
 TEST_F(AlrDetectorTest, ShortSpike) {
   // Start in non-ALR state.
-  EXPECT_FALSE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_FALSE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // Verify that we ALR starts when bitrate drops below 20%.
   SimulateOutgoingTraffic(500, 20);
-  EXPECT_TRUE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_TRUE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // Verify that we stay in ALR region even after a short bitrate spike.
   SimulateOutgoingTraffic(100, 150);
-  EXPECT_TRUE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_TRUE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   SimulateOutgoingTraffic(200, 20);
-  EXPECT_TRUE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_TRUE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // ALR ends when usage is above 50%.
   SimulateOutgoingTraffic(500, 60);
-  EXPECT_FALSE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_FALSE(alr_detector_.GetApplicationLimitedRegionStartTime());
 }
 
 TEST_F(AlrDetectorTest, BandwidthEstimateChanges) {
   // Start in non-ALR state.
-  EXPECT_FALSE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_FALSE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // ALR starts when bitrate drops below 20%.
   SimulateOutgoingTraffic(500, 20);
-  EXPECT_TRUE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_TRUE(alr_detector_.GetApplicationLimitedRegionStartTime());
 
   // When bandwidth estimate drops the detector should stay in ALR mode and quit
   // it shortly afterwards as the sender continues sending the same amount of
   // traffic. This is necessary to ensure that ProbeController can still react
   // to the BWE drop by initiating a new probe.
   alr_detector_.SetEstimatedBitrate(kEstimatedBitrateBps / 5);
-  EXPECT_TRUE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_TRUE(alr_detector_.GetApplicationLimitedRegionStartTime());
   SimulateOutgoingTraffic(10, 20);
-  EXPECT_FALSE(alr_detector_.InApplicationLimitedRegion());
+  EXPECT_FALSE(alr_detector_.GetApplicationLimitedRegionStartTime());
 }
 
 }  // namespace webrtc

webrtc/modules/pacing/mock/mock_paced_sender.h

@@ -33,6 +33,8 @@ class MockPacedSender : public PacedSender {
   MOCK_CONST_METHOD0(QueueInMs, int64_t());
   MOCK_CONST_METHOD0(QueueInPackets, int());
   MOCK_CONST_METHOD0(ExpectedQueueTimeMs, int64_t());
+  MOCK_CONST_METHOD0(GetApplicationLimitedRegionStartTime,
+                     rtc::Optional<int64_t>());
 };
 
 }  // namespace webrtc

webrtc/modules/pacing/paced_sender.cc

@@ -343,9 +343,10 @@ int64_t PacedSender::ExpectedQueueTimeMs() const {
                               pacing_bitrate_kbps_);
 }
 
-bool PacedSender::InApplicationLimitedRegion() const {
+rtc::Optional<int64_t> PacedSender::GetApplicationLimitedRegionStartTime()
+    const {
   CriticalSectionScoped cs(critsect_.get());
-  return alr_detector_->InApplicationLimitedRegion();
+  return alr_detector_->GetApplicationLimitedRegionStartTime();
 }
 
 size_t PacedSender::QueueSizePackets() const {

webrtc/modules/pacing/paced_sender.h

@@ -15,6 +15,7 @@
 #include <memory>
 #include <set>
 
+#include "webrtc/base/optional.h"
 #include "webrtc/base/thread_annotations.h"
 #include "webrtc/modules/include/module.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
@@ -120,12 +121,13 @@ class PacedSender : public Module, public RtpPacketSender {
   // packets in the queue, given the current size and bitrate, ignoring prio.
   virtual int64_t ExpectedQueueTimeMs() const;
 
-  // Application Limited Region refers to operating in a state where the
+  // Returns time in milliseconds when the current application-limited region
+  // started or empty result if the sender is currently not application-limited.
+  //
+  // Application Limited Region (ALR) refers to operating in a state where the
   // traffic on network is limited due to application not having enough
   // traffic to meet the current channel capacity.
-  //
-  // Returns true if network is currently application-limited.
-  bool InApplicationLimitedRegion() const;
+  virtual rtc::Optional<int64_t> GetApplicationLimitedRegionStartTime() const;
 
   // Returns the average time since being enqueued, in milliseconds, for all
   // packets currently in the pacer queue, or 0 if queue is empty.

webrtc/video/video_send_stream.cc

@@ -748,6 +748,9 @@ VideoSendStreamImpl::VideoSendStreamImpl(
   RTC_DCHECK(congestion_controller_);
   RTC_DCHECK(remb_);
 
+  congestion_controller_->EnablePeriodicAlrProbing(
+      config_->periodic_alr_bandwidth_probing);
+
   // RTP/RTCP initialization.
   for (RtpRtcp* rtp_rtcp : rtp_rtcp_modules_) {
     congestion_controller_->packet_router()->AddRtpModule(rtp_rtcp);

webrtc/video_send_stream.h

@@ -185,6 +185,9 @@ class VideoSendStream {
     // stream may send at a rate higher than the estimated available bitrate.
     bool suspend_below_min_bitrate = false;
 
+    // Enables periodic bandwidth probing in application-limited region.
+    bool periodic_alr_bandwidth_probing = false;
+
    private:
     // Access to the copy constructor is private to force use of the Copy()
     // method for those exceptional cases where we do use it.