Add RELATIVE_ARRIVAL_DELAY histogram mode to DelayManager.

- This mode estimates relative packet arrival delay for each incoming packet and adds that value to the histogram.
- The histogram buckets are 20 milliseconds each instead of whole packets.
- The functionality is enabled with a field trial for experimentation.

Bug: webrtc:10333
Change-Id: I8f7499c56802fc1aa1ced2f5310fdd2ef1403515
Reviewed-on: https://webrtc-review.googlesource.com/c/123923
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26871}
diff --git a/modules/audio_coding/neteq/delay_manager.cc b/modules/audio_coding/neteq/delay_manager.cc
index 6147cac..1c7ad19 100644
--- a/modules/audio_coding/neteq/delay_manager.cc
+++ b/modules/audio_coding/neteq/delay_manager.cc
@@ -19,6 +19,7 @@
 
 #include "absl/memory/memory.h"
 #include "modules/audio_coding/neteq/delay_peak_detector.h"
+#include "modules/audio_coding/neteq/histogram.h"
 #include "modules/include/module_common_types_public.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
@@ -39,6 +40,15 @@
 constexpr int kMaxIat = 64;        // Max inter-arrival time to register.
 constexpr int kMaxReorderedPackets =
     10;  // Max number of consecutive reordered packets.
+constexpr int kMaxHistoryPackets =
+    100;  // Max number of packets used to calculate relative packet arrival
+          // delay.
+constexpr int kDelayBuckets = 100;
+constexpr int kBucketSizeMs = 20;
+
+int PercentileToQuantile(double percentile) {
+  return static_cast<int>((1 << 30) * percentile / 100.0 + 0.5);
+}
 
 absl::optional<int> GetForcedLimitProbability() {
   constexpr char kForceTargetDelayPercentileFieldTrial[] =
@@ -52,7 +62,7 @@
     if (sscanf(field_trial_string.c_str(), "Enabled-%lf", &percentile) == 1 &&
         percentile >= 0.0 && percentile <= 100.0) {
       return absl::make_optional<int>(
-          static_cast<int>((1 << 30) * percentile / 100.0 + 0.5));  // in Q30.
+          PercentileToQuantile(percentile));  // in Q30.
     } else {
       RTC_LOG(LS_WARNING) << "Invalid parameter for "
                           << kForceTargetDelayPercentileFieldTrial
@@ -62,19 +72,54 @@
   return absl::nullopt;
 }
 
+struct DelayHistogramConfig {
+  int quantile = 1020054733;  // 0.95 in Q30.
+  int forget_factor = 32745;  // 0.9993 in Q15.
+};
+
+absl::optional<DelayHistogramConfig> GetDelayHistogramConfig() {
+  constexpr char kDelayHistogramFieldTrial[] =
+      "WebRTC-Audio-NetEqDelayHistogram";
+  const bool use_new_delay_manager =
+      webrtc::field_trial::IsEnabled(kDelayHistogramFieldTrial);
+  if (use_new_delay_manager) {
+    const auto field_trial_string =
+        webrtc::field_trial::FindFullName(kDelayHistogramFieldTrial);
+    DelayHistogramConfig config;
+    double percentile = -1.0;
+    double forget_factor = -1.0;
+    if (sscanf(field_trial_string.c_str(), "Enabled-%lf-%lf", &percentile,
+               &forget_factor) == 2 &&
+        percentile >= 0.0 && percentile <= 100.0 && forget_factor >= 0.0 &&
+        forget_factor <= 1.0) {
+      config.quantile = PercentileToQuantile(percentile);
+      config.forget_factor = (1 << 15) * forget_factor;
+    }
+    RTC_LOG(LS_INFO) << "Delay histogram config:"
+                     << " quantile=" << config.quantile
+                     << " forget_factor=" << config.forget_factor;
+    return absl::make_optional(config);
+  }
+  return absl::nullopt;
+}
+
 }  // namespace
 
 namespace webrtc {
 
 DelayManager::DelayManager(size_t max_packets_in_buffer,
                            int base_minimum_delay_ms,
+                           int histogram_quantile,
+                           HistogramMode histogram_mode,
                            bool enable_rtx_handling,
                            DelayPeakDetector* peak_detector,
                            const TickTimer* tick_timer,
-                           std::unique_ptr<Histogram> iat_histogram)
+                           std::unique_ptr<Histogram> histogram)
     : first_packet_received_(false),
       max_packets_in_buffer_(max_packets_in_buffer),
-      iat_histogram_(std::move(iat_histogram)),
+      histogram_(std::move(histogram)),
+      histogram_quantile_(histogram_quantile),
+      histogram_mode_(histogram_mode),
       tick_timer_(tick_timer),
       base_minimum_delay_ms_(base_minimum_delay_ms),
       effective_minimum_delay_ms_(base_minimum_delay_ms),
@@ -92,10 +137,9 @@
       last_pack_cng_or_dtmf_(1),
       frame_length_change_experiment_(
           field_trial::IsEnabled("WebRTC-Audio-NetEqFramelengthExperiment")),
-      forced_limit_probability_(GetForcedLimitProbability()),
       enable_rtx_handling_(enable_rtx_handling) {
   assert(peak_detector);  // Should never be NULL.
-  RTC_CHECK(iat_histogram_);
+  RTC_CHECK(histogram_);
   RTC_DCHECK_GE(base_minimum_delay_ms_, 0);
 
   Reset();
@@ -107,10 +151,24 @@
     bool enable_rtx_handling,
     DelayPeakDetector* peak_detector,
     const TickTimer* tick_timer) {
+  int quantile;
+  std::unique_ptr<Histogram> histogram;
+  HistogramMode mode;
+  auto delay_histogram_config = GetDelayHistogramConfig();
+  if (delay_histogram_config) {
+    DelayHistogramConfig config = delay_histogram_config.value();
+    quantile = config.quantile;
+    histogram =
+        absl::make_unique<Histogram>(kDelayBuckets, config.forget_factor);
+    mode = RELATIVE_ARRIVAL_DELAY;
+  } else {
+    quantile = GetForcedLimitProbability().value_or(kLimitProbability);
+    histogram = absl::make_unique<Histogram>(kMaxIat + 1, kIatFactor);
+    mode = INTER_ARRIVAL_TIME;
+  }
   return absl::make_unique<DelayManager>(
-      max_packets_in_buffer, base_minimum_delay_ms, enable_rtx_handling,
-      peak_detector, tick_timer,
-      absl::make_unique<Histogram>(kMaxIat + 1, kIatFactor));
+      max_packets_in_buffer, base_minimum_delay_ms, quantile, mode,
+      enable_rtx_handling, peak_detector, tick_timer, std::move(histogram));
 }
 
 DelayManager::~DelayManager() {}
@@ -149,30 +207,57 @@
   bool reordered = false;
   if (packet_len_ms > 0) {
     // Cannot update statistics unless |packet_len_ms| is valid.
-    // Calculate inter-arrival time (IAT) in integer "packet times"
-    // (rounding down). This is the value added to the inter-arrival time
-    // histogram.
-    int iat_packets = packet_iat_stopwatch_->ElapsedMs() / packet_len_ms;
-
     if (streaming_mode_) {
       UpdateCumulativeSums(packet_len_ms, sequence_number);
     }
 
+    // Inter-arrival time (IAT) in integer "packet times" (rounding down). This
+    // is the value added to the inter-arrival time histogram.
+    int iat_ms = packet_iat_stopwatch_->ElapsedMs();
+    int iat_packets = iat_ms / packet_len_ms;
     // Check for discontinuous packet sequence and re-ordering.
     if (IsNewerSequenceNumber(sequence_number, last_seq_no_ + 1)) {
       // Compensate for gap in the sequence numbers. Reduce IAT with the
       // expected extra time due to lost packets, but ensure that the IAT is
       // not negative.
-      iat_packets -= static_cast<uint16_t>(sequence_number - last_seq_no_ - 1);
+      int packet_offset =
+          static_cast<uint16_t>(sequence_number - last_seq_no_ - 1);
+      iat_packets -= packet_offset;
       iat_packets = std::max(iat_packets, 0);
+      iat_ms -= packet_offset * packet_len_ms;
+      iat_ms = std::max(iat_ms, 0);
     } else if (!IsNewerSequenceNumber(sequence_number, last_seq_no_)) {
-      iat_packets += static_cast<uint16_t>(last_seq_no_ + 1 - sequence_number);
+      int packet_offset =
+          static_cast<uint16_t>(last_seq_no_ + 1 - sequence_number);
+      iat_packets += packet_offset;
+      iat_ms += packet_offset * packet_len_ms;
       reordered = true;
     }
 
-    // Saturate IAT at maximum value.
-    iat_packets = std::min(iat_packets, iat_histogram_->NumBuckets() - 1);
-    iat_histogram_->Add(iat_packets);
+    switch (histogram_mode_) {
+      case RELATIVE_ARRIVAL_DELAY: {
+        int iat_delay = iat_ms - packet_len_ms;
+        int relative_delay;
+        if (reordered) {
+          relative_delay = std::max(iat_delay, 0);
+        } else {
+          UpdateDelayHistory(iat_delay);
+          relative_delay = CalculateRelativePacketArrivalDelay();
+        }
+        const int index = relative_delay / kBucketSizeMs;
+        if (index < histogram_->NumBuckets()) {
+          // Maximum delay to register is 2000 ms.
+          histogram_->Add(index);
+        }
+        break;
+      }
+      case INTER_ARRIVAL_TIME: {
+        // Saturate IAT at maximum value.
+        iat_packets = std::min(iat_packets, histogram_->NumBuckets() - 1);
+        histogram_->Add(iat_packets);
+        break;
+      }
+    }
     // Calculate new |target_level_| based on updated statistics.
     target_level_ = CalculateTargetLevel(iat_packets, reordered);
     if (streaming_mode_) {
@@ -195,6 +280,26 @@
   return 0;
 }
 
+void DelayManager::UpdateDelayHistory(int iat_delay) {
+  delay_history_.push_back(iat_delay);
+  if (delay_history_.size() > kMaxHistoryPackets) {
+    delay_history_.pop_front();
+  }
+}
+
+int DelayManager::CalculateRelativePacketArrivalDelay() const {
+  // This effectively calculates arrival delay of a packet relative to the
+  // packet preceding the history window. If the arrival delay ever becomes
+  // smaller than zero, it means the reference packet is invalid, and we
+  // move the reference.
+  int relative_delay = 0;
+  for (int delay : delay_history_) {
+    relative_delay += delay;
+    relative_delay = std::max(relative_delay, 0);
+  }
+  return relative_delay;
+}
+
 void DelayManager::UpdateCumulativeSums(int packet_len_ms,
                                         uint16_t sequence_number) {
   // Calculate IAT in Q8, including fractions of a packet (i.e., more
@@ -252,21 +357,30 @@
 }
 
 int DelayManager::CalculateTargetLevel(int iat_packets, bool reordered) {
-  int limit_probability = forced_limit_probability_.value_or(kLimitProbability);
+  int limit_probability = histogram_quantile_;
   if (streaming_mode_) {
     limit_probability = kLimitProbabilityStreaming;
   }
 
-  // Calculate target buffer level from inter-arrival time histogram.
-  // This is the base value for the target buffer level.
-  int target_level = iat_histogram_->Quantile(limit_probability);
-  base_target_level_ = target_level;
-
-  // Update detector for delay peaks.
-  bool delay_peak_found =
-      peak_detector_.Update(iat_packets, reordered, target_level);
-  if (delay_peak_found) {
-    target_level = std::max(target_level, peak_detector_.MaxPeakHeight());
+  int bucket_index = histogram_->Quantile(limit_probability);
+  int target_level;
+  switch (histogram_mode_) {
+    case RELATIVE_ARRIVAL_DELAY: {
+      target_level = 1 + bucket_index * kBucketSizeMs / packet_len_ms_;
+      base_target_level_ = target_level;
+      break;
+    }
+    case INTER_ARRIVAL_TIME: {
+      target_level = bucket_index;
+      base_target_level_ = target_level;
+      // Update detector for delay peaks.
+      bool delay_peak_found =
+          peak_detector_.Update(iat_packets, reordered, target_level);
+      if (delay_peak_found) {
+        target_level = std::max(target_level, peak_detector_.MaxPeakHeight());
+      }
+      break;
+    }
   }
 
   // Sanity check. |target_level| must be strictly positive.
@@ -281,9 +395,10 @@
     RTC_LOG_F(LS_ERROR) << "length_ms = " << length_ms;
     return -1;
   }
-  if (frame_length_change_experiment_ && packet_len_ms_ != length_ms &&
+  if (histogram_mode_ == INTER_ARRIVAL_TIME &&
+      frame_length_change_experiment_ && packet_len_ms_ != length_ms &&
       packet_len_ms_ > 0) {
-    iat_histogram_->Scale(packet_len_ms_, length_ms);
+    histogram_->Scale(packet_len_ms_, length_ms);
   }
 
   packet_len_ms_ = length_ms;
@@ -297,7 +412,7 @@
   packet_len_ms_ = 0;  // Packet size unknown.
   streaming_mode_ = false;
   peak_detector_.Reset();
-  iat_histogram_->Reset();
+  histogram_->Reset();
   base_target_level_ = 4;
   target_level_ = base_target_level_ << 8;
   packet_iat_stopwatch_ = tick_timer_->GetNewStopwatch();
@@ -310,12 +425,12 @@
 double DelayManager::EstimatedClockDriftPpm() const {
   double sum = 0.0;
   // Calculate the expected value based on the probabilities in
-  // |iat_histogram_|.
-  auto buckets = iat_histogram_->buckets();
+  // |histogram_|.
+  auto buckets = histogram_->buckets();
   for (size_t i = 0; i < buckets.size(); ++i) {
     sum += static_cast<double>(buckets[i]) * i;
   }
-  // The probabilities in |iat_histogram_| are in Q30. Divide by 1 << 30 to
+  // The probabilities in |histogram_| are in Q30. Divide by 1 << 30 to
   // convert to Q0; subtract the nominal inter-arrival time (1) to make a zero
   // clockdrift represent as 0; mulitply by 1000000 to produce parts-per-million
   // (ppm).
diff --git a/modules/audio_coding/neteq/delay_manager.h b/modules/audio_coding/neteq/delay_manager.h
index 4f2cb85..11dfeb9 100644
--- a/modules/audio_coding/neteq/delay_manager.h
+++ b/modules/audio_coding/neteq/delay_manager.h
@@ -13,8 +13,8 @@
 
 #include <string.h>  // Provide access to size_t.
 
+#include <deque>
 #include <memory>
-#include <vector>
 
 #include "absl/types/optional.h"
 #include "modules/audio_coding/neteq/histogram.h"
@@ -28,12 +28,19 @@
 
 class DelayManager {
  public:
+  enum HistogramMode {
+    INTER_ARRIVAL_TIME,
+    RELATIVE_ARRIVAL_DELAY,
+  };
+
   DelayManager(size_t max_packets_in_buffer,
                int base_minimum_delay_ms,
+               int histogram_quantile,
+               HistogramMode histogram_mode,
                bool enable_rtx_handling,
                DelayPeakDetector* peak_detector,
                const TickTimer* tick_timer,
-               std::unique_ptr<Histogram> iat_histogram);
+               std::unique_ptr<Histogram> histogram);
 
   // Create a DelayManager object. Notify the delay manager that the packet
   // buffer can hold no more than |max_packets_in_buffer| packets (i.e., this
@@ -117,15 +124,15 @@
   virtual void set_last_pack_cng_or_dtmf(int value);
 
   // This accessor is only intended for testing purposes.
-  const absl::optional<int>& forced_limit_probability_for_test() const {
-    return forced_limit_probability_;
-  }
-
-  // This accessor is only intended for testing purposes.
   int effective_minimum_delay_ms_for_test() const {
     return effective_minimum_delay_ms_;
   }
 
+  // This accessor is only intended for testing purposes.
+  HistogramMode histogram_mode() const { return histogram_mode_; }
+  int histogram_quantile() const { return histogram_quantile_; }
+  int histogram_forget_factor() const { return histogram_->forget_factor(); }
+
  private:
   // Provides value which minimum delay can't exceed based on current buffer
   // size and given |maximum_delay_ms_|. Lower bound is a constant 0.
@@ -134,6 +141,12 @@
   // Provides 75% of currently possible maximum buffer size in milliseconds.
   int MaxBufferTimeQ75() const;
 
+  // Updates |delay_history_|.
+  void UpdateDelayHistory(int iat_delay);
+
+  // Calculate relative packet arrival delay from |delay_history_|.
+  int CalculateRelativePacketArrivalDelay() const;
+
   // Updates |iat_cumulative_sum_| and |max_iat_cumulative_sum_|. (These are
   // used by the streaming mode.) This method is called by Update().
   void UpdateCumulativeSums(int packet_len_ms, uint16_t sequence_number);
@@ -157,7 +170,9 @@
 
   bool first_packet_received_;
   const size_t max_packets_in_buffer_;  // Capacity of the packet buffer.
-  std::unique_ptr<Histogram> iat_histogram_;
+  std::unique_ptr<Histogram> histogram_;
+  const int histogram_quantile_;
+  const HistogramMode histogram_mode_;
   const TickTimer* tick_timer_;
   int base_minimum_delay_ms_;
   // Provides delay which is used by LimitTargetLevel as lower bound on target
@@ -185,9 +200,9 @@
   DelayPeakDetector& peak_detector_;
   int last_pack_cng_or_dtmf_;
   const bool frame_length_change_experiment_;
-  const absl::optional<int> forced_limit_probability_;
   const bool enable_rtx_handling_;
   int num_reordered_packets_ = 0;  // Number of consecutive reordered packets.
+  std::deque<int> delay_history_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(DelayManager);
 };
diff --git a/modules/audio_coding/neteq/delay_manager_unittest.cc b/modules/audio_coding/neteq/delay_manager_unittest.cc
index b3797e2..7b57324 100644
--- a/modules/audio_coding/neteq/delay_manager_unittest.cc
+++ b/modules/audio_coding/neteq/delay_manager_unittest.cc
@@ -25,19 +25,24 @@
 
 namespace webrtc {
 
+namespace {
+constexpr int kMaxNumberOfPackets = 240;
+constexpr int kMinDelayMs = 0;
+constexpr int kTimeStepMs = 10;
+constexpr int kFs = 8000;
+constexpr int kFrameSizeMs = 20;
+constexpr int kTsIncrement = kFrameSizeMs * kFs / 1000;
+constexpr int kMaxBufferSizeMs = kMaxNumberOfPackets * kFrameSizeMs;
+constexpr int kDefaultHistogramQuantile = 1020054733;
+constexpr int kMaxIat = 64;
+constexpr int kForgetFactor = 32745;
+}  // namespace
+
 using ::testing::Return;
 using ::testing::_;
 
 class DelayManagerTest : public ::testing::Test {
  protected:
-  static const int kMaxNumberOfPackets = 240;
-  static const int kMinDelayMs = 0;
-  static const int kTimeStepMs = 10;
-  static const int kFs = 8000;
-  static const int kFrameSizeMs = 20;
-  static const int kTsIncrement = kFrameSizeMs * kFs / 1000;
-  static const int kMaxBufferSizeMs = kMaxNumberOfPackets * kFrameSizeMs;
-
   DelayManagerTest();
   virtual void SetUp();
   virtual void TearDown();
@@ -49,11 +54,13 @@
   std::unique_ptr<DelayManager> dm_;
   TickTimer tick_timer_;
   MockDelayPeakDetector detector_;
-  bool use_mock_histogram_ = false;
   MockHistogram* mock_histogram_;
   uint16_t seq_no_;
   uint32_t ts_;
   bool enable_rtx_handling_ = false;
+  bool use_mock_histogram_ = false;
+  DelayManager::HistogramMode histogram_mode_ =
+      DelayManager::HistogramMode::INTER_ARRIVAL_TIME;
 };
 
 DelayManagerTest::DelayManagerTest()
@@ -68,18 +75,17 @@
 
 void DelayManagerTest::RecreateDelayManager() {
   EXPECT_CALL(detector_, Reset()).Times(1);
-  std::unique_ptr<Histogram> histogram;
-  static const int kMaxIat = 64;
-  static const int kForgetFactor = 32745;
   if (use_mock_histogram_) {
     mock_histogram_ = new MockHistogram(kMaxIat, kForgetFactor);
-    histogram.reset(mock_histogram_);
+    std::unique_ptr<Histogram> histogram(mock_histogram_);
+    dm_ = absl::make_unique<DelayManager>(
+        kMaxNumberOfPackets, kMinDelayMs, kDefaultHistogramQuantile,
+        histogram_mode_, enable_rtx_handling_, &detector_, &tick_timer_,
+        std::move(histogram));
   } else {
-    histogram = absl::make_unique<Histogram>(kMaxIat, kForgetFactor);
+    dm_ = DelayManager::Create(kMaxNumberOfPackets, kMinDelayMs,
+                               enable_rtx_handling_, &detector_, &tick_timer_);
   }
-  dm_.reset(new DelayManager(kMaxNumberOfPackets, kMinDelayMs,
-                             enable_rtx_handling_, &detector_, &tick_timer_,
-                             std::move(histogram)));
 }
 
 void DelayManagerTest::SetPacketAudioLength(int lengt_ms) {
@@ -577,8 +583,7 @@
   test::ScopedFieldTrials field_trial(
       "WebRTC-Audio-NetEqForceTargetDelayPercentile/Enabled-0/");
   RecreateDelayManager();
-  EXPECT_EQ(absl::make_optional<int>(0),
-            dm_->forced_limit_probability_for_test());
+  EXPECT_EQ(0, dm_->histogram_quantile());
 
   SetPacketAudioLength(kFrameSizeMs);
   // First packet arrival.
@@ -599,33 +604,109 @@
     test::ScopedFieldTrials field_trial(
         "WebRTC-Audio-NetEqForceTargetDelayPercentile/Enabled-95/");
     RecreateDelayManager();
-    EXPECT_EQ(absl::make_optional<int>(1020054733),
-              dm_->forced_limit_probability_for_test());  // 1/20 in Q30
+    EXPECT_EQ(kDefaultHistogramQuantile, dm_->histogram_quantile());
   }
   {
     test::ScopedFieldTrials field_trial(
         "WebRTC-Audio-NetEqForceTargetDelayPercentile/Enabled-99.95/");
     RecreateDelayManager();
-    EXPECT_EQ(absl::make_optional<int>(1073204953),
-              dm_->forced_limit_probability_for_test());  // 1/2000 in Q30
+    EXPECT_EQ(1073204953, dm_->histogram_quantile());  // 0.9995 in Q30.
   }
   {
     test::ScopedFieldTrials field_trial(
         "WebRTC-Audio-NetEqForceTargetDelayPercentile/Disabled/");
     RecreateDelayManager();
-    EXPECT_EQ(absl::nullopt, dm_->forced_limit_probability_for_test());
+    EXPECT_EQ(kDefaultHistogramQuantile, dm_->histogram_quantile());
   }
   {
     test::ScopedFieldTrials field_trial(
         "WebRTC-Audio-NetEqForceTargetDelayPercentile/Enabled--1/");
-    EXPECT_EQ(absl::nullopt, dm_->forced_limit_probability_for_test());
+    EXPECT_EQ(kDefaultHistogramQuantile, dm_->histogram_quantile());
   }
   {
     test::ScopedFieldTrials field_trial(
         "WebRTC-Audio-NetEqForceTargetDelayPercentile/Enabled-100.1/");
     RecreateDelayManager();
-    EXPECT_EQ(absl::nullopt, dm_->forced_limit_probability_for_test());
+    EXPECT_EQ(kDefaultHistogramQuantile, dm_->histogram_quantile());
   }
 }
 
+TEST_F(DelayManagerTest, DelayHistogramFieldTrial) {
+  {
+    test::ScopedFieldTrials field_trial(
+        "WebRTC-Audio-NetEqDelayHistogram/Enabled-96-0.998/");
+    RecreateDelayManager();
+    EXPECT_EQ(DelayManager::HistogramMode::RELATIVE_ARRIVAL_DELAY,
+              dm_->histogram_mode());
+    EXPECT_EQ(1030792151, dm_->histogram_quantile());  // 0.96 in Q30.
+    EXPECT_EQ(32702, dm_->histogram_forget_factor());  // 0.998 in Q15.
+  }
+  {
+    test::ScopedFieldTrials field_trial(
+        "WebRTC-Audio-NetEqDelayHistogram/Enabled-97.5-0.998/");
+    RecreateDelayManager();
+    EXPECT_EQ(DelayManager::HistogramMode::RELATIVE_ARRIVAL_DELAY,
+              dm_->histogram_mode());
+    EXPECT_EQ(1046898278, dm_->histogram_quantile());  // 0.975 in Q30.
+    EXPECT_EQ(32702, dm_->histogram_forget_factor());  // 0.998 in Q15.
+  }
+  {
+    // NetEqDelayHistogram should take precedence over
+    // NetEqForceTargetDelayPercentile.
+    test::ScopedFieldTrials field_trial(
+        "WebRTC-Audio-NetEqForceTargetDelayPercentile/Enabled-99.95/"
+        "WebRTC-Audio-NetEqDelayHistogram/Enabled-96-0.998/");
+    RecreateDelayManager();
+    EXPECT_EQ(DelayManager::HistogramMode::RELATIVE_ARRIVAL_DELAY,
+              dm_->histogram_mode());
+    EXPECT_EQ(1030792151, dm_->histogram_quantile());  // 0.96 in Q30.
+    EXPECT_EQ(32702, dm_->histogram_forget_factor());  // 0.998 in Q15.
+  }
+  {
+    // Invalid parameters.
+    test::ScopedFieldTrials field_trial(
+        "WebRTC-Audio-NetEqDelayHistogram/Enabled-96/");
+    RecreateDelayManager();
+    EXPECT_EQ(DelayManager::HistogramMode::RELATIVE_ARRIVAL_DELAY,
+              dm_->histogram_mode());
+    EXPECT_EQ(kDefaultHistogramQuantile,
+              dm_->histogram_quantile());                      // 0.95 in Q30.
+    EXPECT_EQ(kForgetFactor, dm_->histogram_forget_factor());  // 0.9993 in Q15.
+  }
+  {
+    test::ScopedFieldTrials field_trial(
+        "WebRTC-Audio-NetEqDelayHistogram/Disabled/");
+    RecreateDelayManager();
+    EXPECT_EQ(DelayManager::HistogramMode::INTER_ARRIVAL_TIME,
+              dm_->histogram_mode());
+    EXPECT_EQ(kDefaultHistogramQuantile,
+              dm_->histogram_quantile());                      // 0.95 in Q30.
+    EXPECT_EQ(kForgetFactor, dm_->histogram_forget_factor());  // 0.9993 in Q15.
+  }
+}
+
+TEST_F(DelayManagerTest, RelativeArrivalDelayMode) {
+  histogram_mode_ = DelayManager::HistogramMode::RELATIVE_ARRIVAL_DELAY;
+  use_mock_histogram_ = true;
+  RecreateDelayManager();
+
+  SetPacketAudioLength(kFrameSizeMs);
+  InsertNextPacket();
+
+  IncreaseTime(kFrameSizeMs);
+  EXPECT_CALL(*mock_histogram_, Add(0));  // Not delayed.
+  InsertNextPacket();
+
+  IncreaseTime(2 * kFrameSizeMs);
+  EXPECT_CALL(*mock_histogram_, Add(1));  // 20ms delayed.
+  EXPECT_EQ(0, dm_->Update(seq_no_, ts_, kFs));
+
+  IncreaseTime(2 * kFrameSizeMs);
+  EXPECT_CALL(*mock_histogram_, Add(2));  // 40ms delayed.
+  EXPECT_EQ(0, dm_->Update(seq_no_ + 1, ts_ + kTsIncrement, kFs));
+
+  EXPECT_CALL(*mock_histogram_, Add(1));  // Reordered, 20ms delayed.
+  EXPECT_EQ(0, dm_->Update(seq_no_, ts_, kFs));
+}
+
 }  // namespace webrtc
diff --git a/modules/audio_coding/neteq/histogram.h b/modules/audio_coding/neteq/histogram.h
index 66b4fe9..fc8f612 100644
--- a/modules/audio_coding/neteq/histogram.h
+++ b/modules/audio_coding/neteq/histogram.h
@@ -43,6 +43,8 @@
   // Returns the probability for each bucket in Q30.
   std::vector<int> buckets() const { return buckets_; }
 
+  int forget_factor() const { return base_forget_factor_; }
+
   // Made public for testing.
   static std::vector<int> ScaleBuckets(const std::vector<int>& buckets,
                                        int old_bucket_width,
diff --git a/modules/audio_coding/neteq/mock/mock_delay_manager.h b/modules/audio_coding/neteq/mock/mock_delay_manager.h
index fa07e20..63dd575 100644
--- a/modules/audio_coding/neteq/mock/mock_delay_manager.h
+++ b/modules/audio_coding/neteq/mock/mock_delay_manager.h
@@ -23,12 +23,16 @@
  public:
   MockDelayManager(size_t max_packets_in_buffer,
                    int base_min_target_delay_ms,
+                   int histogram_quantile,
+                   HistogramMode histogram_mode,
                    bool enable_rtx_handling,
                    DelayPeakDetector* peak_detector,
                    const TickTimer* tick_timer,
                    std::unique_ptr<Histogram> histogram)
       : DelayManager(max_packets_in_buffer,
                      base_min_target_delay_ms,
+                     histogram_quantile,
+                     histogram_mode,
                      enable_rtx_handling,
                      peak_detector,
                      tick_timer,
diff --git a/modules/audio_coding/neteq/neteq_impl_unittest.cc b/modules/audio_coding/neteq/neteq_impl_unittest.cc
index e7552c1..86fbe9c 100644
--- a/modules/audio_coding/neteq/neteq_impl_unittest.cc
+++ b/modules/audio_coding/neteq/neteq_impl_unittest.cc
@@ -97,7 +97,8 @@
 
     if (use_mock_delay_manager_) {
       std::unique_ptr<MockDelayManager> mock(new MockDelayManager(
-          config_.max_packets_in_buffer, config_.min_delay_ms,
+          config_.max_packets_in_buffer, config_.min_delay_ms, 1020054733,
+          DelayManager::HistogramMode::INTER_ARRIVAL_TIME,
           config_.enable_rtx_handling, delay_peak_detector_, tick_timer_,
           absl::make_unique<Histogram>(50, 32745)));
       mock_delay_manager_ = mock.get();