1 files changed, 763 insertions, 0 deletions
diff --git a/third_party/libwebrtc/modules/pacing/task_queue_paced_sender_unittest.cc b/third_party/libwebrtc/modules/pacing/task_queue_paced_sender_unittest.cc
new file mode 100644
index 0000000000..54347493e7
--- /dev/null
+++ b/third_party/libwebrtc/modules/pacing/task_queue_paced_sender_unittest.cc
@@ -0,0 +1,763 @@
+/*
+ *  Copyright (c) 2019 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "modules/pacing/task_queue_paced_sender.h"
+
+#include <algorithm>
+#include <atomic>
+#include <list>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "api/task_queue/task_queue_base.h"
+#include "api/task_queue/task_queue_factory.h"
+#include "api/transport/network_types.h"
+#include "api/units/data_rate.h"
+#include "api/units/data_size.h"
+#include "api/units/time_delta.h"
+#include "modules/pacing/packet_router.h"
+#include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
+#include "test/gmock.h"
+#include "test/gtest.h"
+#include "test/scoped_key_value_config.h"
+#include "test/time_controller/simulated_time_controller.h"
+
+using ::testing::_;
+using ::testing::AtLeast;
+using ::testing::Return;
+using ::testing::SaveArg;
+
+namespace webrtc {
+namespace {
+constexpr uint32_t kAudioSsrc = 12345;
+constexpr uint32_t kVideoSsrc = 234565;
+constexpr uint32_t kVideoRtxSsrc = 34567;
+constexpr uint32_t kFlexFecSsrc = 45678;
+constexpr size_t kDefaultPacketSize = 1234;
+
+class MockPacketRouter : public PacketRouter {
+ public:
+  MOCK_METHOD(void,
+              SendPacket,
+              (std::unique_ptr<RtpPacketToSend> packet,
+               const PacedPacketInfo& cluster_info),
+              (override));
+  MOCK_METHOD(std::vector<std::unique_ptr<RtpPacketToSend>>,
+              FetchFec,
+              (),
+              (override));
+  MOCK_METHOD(std::vector<std::unique_ptr<RtpPacketToSend>>,
+              GeneratePadding,
+              (DataSize target_size),
+              (override));
+};
+
+std::vector<std::unique_ptr<RtpPacketToSend>> GeneratePadding(
+    DataSize target_size) {
+  // 224 bytes is the max padding size for plain padding packets generated by
+  // RTPSender::GeneratePadding().
+  const DataSize kMaxPaddingPacketSize = DataSize::Bytes(224);
+  DataSize padding_generated = DataSize::Zero();
+  std::vector<std::unique_ptr<RtpPacketToSend>> padding_packets;
+  while (padding_generated < target_size) {
+    DataSize packet_size =
+        std::min(target_size - padding_generated, kMaxPaddingPacketSize);
+    padding_generated += packet_size;
+    auto padding_packet =
+        std::make_unique<RtpPacketToSend>(/*extensions=*/nullptr);
+    padding_packet->set_packet_type(RtpPacketMediaType::kPadding);
+    padding_packet->SetPadding(packet_size.bytes());
+    padding_packets.push_back(std::move(padding_packet));
+  }
+  return padding_packets;
+}
+
+}  // namespace
+
+namespace test {
+
+std::unique_ptr<RtpPacketToSend> BuildRtpPacket(RtpPacketMediaType type) {
+  auto packet = std::make_unique<RtpPacketToSend>(nullptr);
+  packet->set_packet_type(type);
+  switch (type) {
+    case RtpPacketMediaType::kAudio:
+      packet->SetSsrc(kAudioSsrc);
+      break;
+    case RtpPacketMediaType::kVideo:
+      packet->SetSsrc(kVideoSsrc);
+      break;
+    case RtpPacketMediaType::kRetransmission:
+    case RtpPacketMediaType::kPadding:
+      packet->SetSsrc(kVideoRtxSsrc);
+      break;
+    case RtpPacketMediaType::kForwardErrorCorrection:
+      packet->SetSsrc(kFlexFecSsrc);
+      break;
+  }
+
+  packet->SetPayloadSize(kDefaultPacketSize);
+  return packet;
+}
+
+std::vector<std::unique_ptr<RtpPacketToSend>> GeneratePackets(
+    RtpPacketMediaType type,
+    size_t num_packets) {
+  std::vector<std::unique_ptr<RtpPacketToSend>> packets;
+  for (size_t i = 0; i < num_packets; ++i) {
+    packets.push_back(BuildRtpPacket(type));
+  }
+  return packets;
+}
+
+TEST(TaskQueuePacedSenderTest, PacesPackets) {
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Insert a number of packets, covering one second.
+  static constexpr size_t kPacketsToSend = 42;
+  SequenceChecker sequence_checker;
+  pacer.SetPacingRates(
+      DataRate::BitsPerSec(kDefaultPacketSize * 8 * kPacketsToSend),
+      DataRate::Zero());
+  pacer.EnsureStarted();
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kPacketsToSend));
+
+  // Expect all of them to be sent.
+  size_t packets_sent = 0;
+  Timestamp end_time = Timestamp::PlusInfinity();
+  EXPECT_CALL(packet_router, SendPacket)
+      .WillRepeatedly([&](std::unique_ptr<RtpPacketToSend> packet,
+                          const PacedPacketInfo& cluster_info) {
+        ++packets_sent;
+        if (packets_sent == kPacketsToSend) {
+          end_time = time_controller.GetClock()->CurrentTime();
+        }
+        EXPECT_TRUE(sequence_checker.IsCurrent());
+      });
+
+  const Timestamp start_time = time_controller.GetClock()->CurrentTime();
+
+  // Packets should be sent over a period of close to 1s. Expect a little
+  // lower than this since initial probing is a bit quicker.
+  time_controller.AdvanceTime(TimeDelta::Seconds(1));
+  EXPECT_EQ(packets_sent, kPacketsToSend);
+  ASSERT_TRUE(end_time.IsFinite());
+  EXPECT_NEAR((end_time - start_time).ms<double>(), 1000.0, 50.0);
+}
+
+// Same test as above, but with 0.5s of burst applied.
+TEST(TaskQueuePacedSenderTest, PacesPacketsWithBurst) {
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback,
+                             // Half a second of bursting.
+                             TimeDelta::Seconds(0.5));
+
+  // Insert a number of packets, covering one second.
+  static constexpr size_t kPacketsToSend = 42;
+  SequenceChecker sequence_checker;
+  pacer.SetPacingRates(
+      DataRate::BitsPerSec(kDefaultPacketSize * 8 * kPacketsToSend),
+      DataRate::Zero());
+  pacer.EnsureStarted();
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kPacketsToSend));
+
+  // Expect all of them to be sent.
+  size_t packets_sent = 0;
+  Timestamp end_time = Timestamp::PlusInfinity();
+  EXPECT_CALL(packet_router, SendPacket)
+      .WillRepeatedly([&](std::unique_ptr<RtpPacketToSend> packet,
+                          const PacedPacketInfo& cluster_info) {
+        ++packets_sent;
+        if (packets_sent == kPacketsToSend) {
+          end_time = time_controller.GetClock()->CurrentTime();
+        }
+        EXPECT_TRUE(sequence_checker.IsCurrent());
+      });
+
+  const Timestamp start_time = time_controller.GetClock()->CurrentTime();
+
+  // Packets should be sent over a period of close to 1s. Expect a little
+  // lower than this since initial probing is a bit quicker.
+  time_controller.AdvanceTime(TimeDelta::Seconds(1));
+  EXPECT_EQ(packets_sent, kPacketsToSend);
+  ASSERT_TRUE(end_time.IsFinite());
+  // Because of half a second of burst, what would normally have been paced over
+  // ~1 second now takes ~0.5 seconds.
+  EXPECT_NEAR((end_time - start_time).ms<double>(), 500.0, 50.0);
+}
+
+TEST(TaskQueuePacedSenderTest, ReschedulesProcessOnRateChange) {
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Insert a number of packets to be sent 200ms apart.
+  const size_t kPacketsPerSecond = 5;
+  const DataRate kPacingRate =
+      DataRate::BitsPerSec(kDefaultPacketSize * 8 * kPacketsPerSecond);
+  pacer.SetPacingRates(kPacingRate, DataRate::Zero());
+  pacer.EnsureStarted();
+
+  // Send some initial packets to be rid of any probes.
+  EXPECT_CALL(packet_router, SendPacket).Times(kPacketsPerSecond);
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kPacketsPerSecond));
+  time_controller.AdvanceTime(TimeDelta::Seconds(1));
+
+  // Insert three packets, and record send time of each of them.
+  // After the second packet is sent, double the send rate so we can
+  // check the third packets is sent after half the wait time.
+  Timestamp first_packet_time = Timestamp::MinusInfinity();
+  Timestamp second_packet_time = Timestamp::MinusInfinity();
+  Timestamp third_packet_time = Timestamp::MinusInfinity();
+
+  EXPECT_CALL(packet_router, SendPacket)
+      .Times(3)
+      .WillRepeatedly([&](std::unique_ptr<RtpPacketToSend> packet,
+                          const PacedPacketInfo& cluster_info) {
+        if (first_packet_time.IsInfinite()) {
+          first_packet_time = time_controller.GetClock()->CurrentTime();
+        } else if (second_packet_time.IsInfinite()) {
+          second_packet_time = time_controller.GetClock()->CurrentTime();
+          // Avoid invoke SetPacingRate in the context of sending a packet.
+          time_controller.GetMainThread()->PostTask(
+              [&] { pacer.SetPacingRates(2 * kPacingRate, DataRate::Zero()); });
+        } else {
+          third_packet_time = time_controller.GetClock()->CurrentTime();
+        }
+      });
+
+  pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 3));
+  time_controller.AdvanceTime(TimeDelta::Millis(500));
+  ASSERT_TRUE(third_packet_time.IsFinite());
+  EXPECT_NEAR((second_packet_time - first_packet_time).ms<double>(), 200.0,
+              1.0);
+  EXPECT_NEAR((third_packet_time - second_packet_time).ms<double>(), 100.0,
+              1.0);
+}
+
+TEST(TaskQueuePacedSenderTest, SendsAudioImmediately) {
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  const DataRate kPacingDataRate = DataRate::KilobitsPerSec(125);
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = kPacketSize / kPacingDataRate;
+
+  pacer.SetPacingRates(kPacingDataRate, DataRate::Zero());
+  pacer.EnsureStarted();
+
+  // Add some initial video packets, only one should be sent.
+  EXPECT_CALL(packet_router, SendPacket);
+  pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 10));
+  time_controller.AdvanceTime(TimeDelta::Zero());
+  ::testing::Mock::VerifyAndClearExpectations(&packet_router);
+
+  // Advance time, but still before next packet should be sent.
+  time_controller.AdvanceTime(kPacketPacingTime / 2);
+
+  // Insert an audio packet, it should be sent immediately.
+  EXPECT_CALL(packet_router, SendPacket);
+  pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kAudio, 1));
+  time_controller.AdvanceTime(TimeDelta::Zero());
+  ::testing::Mock::VerifyAndClearExpectations(&packet_router);
+}
+
+TEST(TaskQueuePacedSenderTest, SleepsDuringCoalscingWindow) {
+  const TimeDelta kCoalescingWindow = TimeDelta::Millis(5);
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             kCoalescingWindow,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Set rates so one packet adds one ms of buffer level.
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = TimeDelta::Millis(1);
+  const DataRate kPacingDataRate = kPacketSize / kPacketPacingTime;
+
+  pacer.SetPacingRates(kPacingDataRate, DataRate::Zero());
+  pacer.EnsureStarted();
+
+  // Add 10 packets. The first should be sent immediately since the buffers
+  // are clear.
+  EXPECT_CALL(packet_router, SendPacket);
+  pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 10));
+  time_controller.AdvanceTime(TimeDelta::Zero());
+  ::testing::Mock::VerifyAndClearExpectations(&packet_router);
+
+  // Advance time to 1ms before the coalescing window ends. No packets should
+  // be sent.
+  EXPECT_CALL(packet_router, SendPacket).Times(0);
+  time_controller.AdvanceTime(kCoalescingWindow - TimeDelta::Millis(1));
+
+  // Advance time to where coalescing window ends. All packets that should
+  // have been sent up til now will be sent.
+  EXPECT_CALL(packet_router, SendPacket).Times(5);
+  time_controller.AdvanceTime(TimeDelta::Millis(1));
+  ::testing::Mock::VerifyAndClearExpectations(&packet_router);
+}
+
+TEST(TaskQueuePacedSenderTest, ProbingOverridesCoalescingWindow) {
+  const TimeDelta kCoalescingWindow = TimeDelta::Millis(5);
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             kCoalescingWindow,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Set rates so one packet adds one ms of buffer level.
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = TimeDelta::Millis(1);
+  const DataRate kPacingDataRate = kPacketSize / kPacketPacingTime;
+
+  pacer.SetPacingRates(kPacingDataRate, DataRate::Zero());
+  pacer.EnsureStarted();
+
+  // Add 10 packets. The first should be sent immediately since the buffers
+  // are clear. This will also trigger the probe to start.
+  EXPECT_CALL(packet_router, SendPacket).Times(AtLeast(1));
+  pacer.CreateProbeClusters(
+      {{.at_time = time_controller.GetClock()->CurrentTime(),
+        .target_data_rate = kPacingDataRate * 2,
+        .target_duration = TimeDelta::Millis(15),
+        .target_probe_count = 5,
+        .id = 17}});
+  pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 10));
+  time_controller.AdvanceTime(TimeDelta::Zero());
+  ::testing::Mock::VerifyAndClearExpectations(&packet_router);
+
+  // Advance time to 1ms before the coalescing window ends. Packets should be
+  // flying.
+  EXPECT_CALL(packet_router, SendPacket).Times(AtLeast(1));
+  time_controller.AdvanceTime(kCoalescingWindow - TimeDelta::Millis(1));
+}
+
+TEST(TaskQueuePacedSenderTest, SchedulesProbeAtSentTime) {
+  ScopedKeyValueConfig trials(
+      "WebRTC-Bwe-ProbingBehavior/min_probe_delta:1ms/");
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Set rates so one packet adds 4ms of buffer level.
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = TimeDelta::Millis(4);
+  const DataRate kPacingDataRate = kPacketSize / kPacketPacingTime;
+  pacer.SetPacingRates(kPacingDataRate, /*padding_rate=*/DataRate::Zero());
+  pacer.EnsureStarted();
+  EXPECT_CALL(packet_router, FetchFec).WillRepeatedly([]() {
+    return std::vector<std::unique_ptr<RtpPacketToSend>>();
+  });
+  EXPECT_CALL(packet_router, GeneratePadding(_))
+      .WillRepeatedly(
+          [](DataSize target_size) { return GeneratePadding(target_size); });
+
+  // Enqueue two packets, only the first is sent immediately and the next
+  // will be scheduled for sending in 4ms.
+  pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 2));
+  const int kNotAProbe = PacedPacketInfo::kNotAProbe;
+  EXPECT_CALL(packet_router,
+              SendPacket(_, ::testing::Field(&PacedPacketInfo::probe_cluster_id,
+                                             kNotAProbe)));
+  // Advance to less than 3ms before next packet send time.
+  time_controller.AdvanceTime(TimeDelta::Micros(1001));
+
+  // Trigger a probe at 2x the current pacing rate and insert the number of
+  // packets the probe needs.
+  const DataRate kProbeRate = 2 * kPacingDataRate;
+  const int kProbeClusterId = 1;
+  pacer.CreateProbeClusters(
+      {{.at_time = time_controller.GetClock()->CurrentTime(),
+        .target_data_rate = kProbeRate,
+        .target_duration = TimeDelta::Millis(15),
+        .target_probe_count = 4,
+        .id = kProbeClusterId}});
+
+  // Expected size for each probe in a cluster is twice the expected bits sent
+  // during min_probe_delta.
+  // Expect one additional call since probe always starts with a small (1 byte)
+  // padding packet that's not counted into the probe rate here.
+  const TimeDelta kProbeTimeDelta = TimeDelta::Millis(2);
+  const DataSize kProbeSize = kProbeRate * kProbeTimeDelta;
+  const size_t kNumPacketsInProbe =
+      (kProbeSize + kPacketSize - DataSize::Bytes(1)) / kPacketSize;
+  EXPECT_CALL(packet_router,
+              SendPacket(_, ::testing::Field(&PacedPacketInfo::probe_cluster_id,
+                                             kProbeClusterId)))
+      .Times(kNumPacketsInProbe + 1);
+
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kNumPacketsInProbe));
+  time_controller.AdvanceTime(TimeDelta::Zero());
+
+  // The pacer should have scheduled the next probe to be sent in
+  // kProbeTimeDelta. That there was existing scheduled call less than
+  // PacingController::kMinSleepTime before this should not matter.
+  EXPECT_CALL(packet_router,
+              SendPacket(_, ::testing::Field(&PacedPacketInfo::probe_cluster_id,
+                                             kProbeClusterId)))
+      .Times(AtLeast(1));
+  time_controller.AdvanceTime(TimeDelta::Millis(2));
+}
+
+TEST(TaskQueuePacedSenderTest, NoMinSleepTimeWhenProbing) {
+  // Set min_probe_delta to be less than kMinSleepTime (1ms).
+  const TimeDelta kMinProbeDelta = TimeDelta::Micros(200);
+  ScopedKeyValueConfig trials(
+      "WebRTC-Bwe-ProbingBehavior/min_probe_delta:200us/");
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Set rates so one packet adds 4ms of buffer level.
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = TimeDelta::Millis(4);
+  const DataRate kPacingDataRate = kPacketSize / kPacketPacingTime;
+  pacer.SetPacingRates(kPacingDataRate, /*padding_rate=*/DataRate::Zero());
+  pacer.EnsureStarted();
+  EXPECT_CALL(packet_router, FetchFec).WillRepeatedly([]() {
+    return std::vector<std::unique_ptr<RtpPacketToSend>>();
+  });
+  EXPECT_CALL(packet_router, GeneratePadding)
+      .WillRepeatedly(
+          [](DataSize target_size) { return GeneratePadding(target_size); });
+
+  // Set a high probe rate.
+  const int kProbeClusterId = 1;
+  DataRate kProbingRate = kPacingDataRate * 10;
+
+  pacer.CreateProbeClusters(
+      {{.at_time = time_controller.GetClock()->CurrentTime(),
+        .target_data_rate = kProbingRate,
+        .target_duration = TimeDelta::Millis(15),
+        .target_probe_count = 5,
+        .id = kProbeClusterId}});
+
+  // Advance time less than PacingController::kMinSleepTime, probing packets
+  // for the first millisecond should be sent immediately. Min delta between
+  // probes is 200us, meaning 4 times per ms we will get least one call to
+  // SendPacket().
+  DataSize data_sent = DataSize::Zero();
+  EXPECT_CALL(packet_router,
+              SendPacket(_, ::testing::Field(&PacedPacketInfo::probe_cluster_id,
+                                             kProbeClusterId)))
+      .Times(AtLeast(4))
+      .WillRepeatedly([&](std::unique_ptr<RtpPacketToSend> packet,
+                          const PacedPacketInfo&) {
+        data_sent +=
+            DataSize::Bytes(packet->payload_size() + packet->padding_size());
+      });
+
+  // Add one packet to kickstart probing, the rest will be padding packets.
+  pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 1));
+  time_controller.AdvanceTime(kMinProbeDelta);
+
+  // Verify the amount of probing data sent.
+  // Probe always starts with a small (1 byte) padding packet that's not
+  // counted into the probe rate here.
+  const DataSize kMinProbeSize = kMinProbeDelta * kProbingRate;
+  EXPECT_EQ(data_sent, DataSize::Bytes(1) + kPacketSize + 4 * kMinProbeSize);
+}
+
+TEST(TaskQueuePacedSenderTest, PacketBasedCoalescing) {
+  const TimeDelta kFixedCoalescingWindow = TimeDelta::Millis(10);
+  const int kPacketBasedHoldback = 5;
+
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             kFixedCoalescingWindow, kPacketBasedHoldback);
+
+  // Set rates so one packet adds one ms of buffer level.
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = TimeDelta::Millis(1);
+  const DataRate kPacingDataRate = kPacketSize / kPacketPacingTime;
+  const TimeDelta kExpectedHoldbackWindow =
+      kPacketPacingTime * kPacketBasedHoldback;
+  // `kFixedCoalescingWindow` sets the upper bound for the window.
+  ASSERT_GE(kFixedCoalescingWindow, kExpectedHoldbackWindow);
+
+  pacer.SetPacingRates(kPacingDataRate, DataRate::Zero());
+  EXPECT_CALL(packet_router, FetchFec).WillRepeatedly([]() {
+    return std::vector<std::unique_ptr<RtpPacketToSend>>();
+  });
+  pacer.EnsureStarted();
+
+  // Add some packets and wait till all have been sent, so that the pacer
+  // has a valid estimate of packet size.
+  const int kNumWarmupPackets = 40;
+  EXPECT_CALL(packet_router, SendPacket).Times(kNumWarmupPackets);
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kNumWarmupPackets));
+  // Wait until all packes have been sent, with a 2x margin.
+  time_controller.AdvanceTime(kPacketPacingTime * (kNumWarmupPackets * 2));
+
+  // Enqueue packets. Expect only the first one to be sent immediately.
+  EXPECT_CALL(packet_router, SendPacket).Times(1);
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kPacketBasedHoldback));
+  time_controller.AdvanceTime(TimeDelta::Zero());
+
+  // Advance time to 1ms before the coalescing window ends.
+  EXPECT_CALL(packet_router, SendPacket).Times(0);
+  time_controller.AdvanceTime(kExpectedHoldbackWindow - TimeDelta::Millis(1));
+
+  // Advance past where the coalescing window should end.
+  EXPECT_CALL(packet_router, SendPacket).Times(kPacketBasedHoldback - 1);
+  time_controller.AdvanceTime(TimeDelta::Millis(1));
+}
+
+TEST(TaskQueuePacedSenderTest, FixedHoldBackHasPriorityOverPackets) {
+  const TimeDelta kFixedCoalescingWindow = TimeDelta::Millis(2);
+  const int kPacketBasedHoldback = 5;
+
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             kFixedCoalescingWindow, kPacketBasedHoldback);
+
+  // Set rates so one packet adds one ms of buffer level.
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = TimeDelta::Millis(1);
+  const DataRate kPacingDataRate = kPacketSize / kPacketPacingTime;
+  const TimeDelta kExpectedPacketHoldbackWindow =
+      kPacketPacingTime * kPacketBasedHoldback;
+  // |kFixedCoalescingWindow| sets the upper bound for the window.
+  ASSERT_LT(kFixedCoalescingWindow, kExpectedPacketHoldbackWindow);
+
+  pacer.SetPacingRates(kPacingDataRate, DataRate::Zero());
+  EXPECT_CALL(packet_router, FetchFec).WillRepeatedly([]() {
+    return std::vector<std::unique_ptr<RtpPacketToSend>>();
+  });
+  pacer.EnsureStarted();
+
+  // Add some packets and wait till all have been sent, so that the pacer
+  // has a valid estimate of packet size.
+  const int kNumWarmupPackets = 40;
+  EXPECT_CALL(packet_router, SendPacket).Times(kNumWarmupPackets);
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kNumWarmupPackets));
+  // Wait until all packes have been sent, with a 2x margin.
+  time_controller.AdvanceTime(kPacketPacingTime * (kNumWarmupPackets * 2));
+
+  // Enqueue packets. Expect onlt the first one to be sent immediately.
+  EXPECT_CALL(packet_router, SendPacket).Times(1);
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kPacketBasedHoldback));
+  time_controller.AdvanceTime(TimeDelta::Zero());
+
+  // Advance time to the fixed coalescing window, that should take presedence so
+  // at least some of the packets should be sent.
+  EXPECT_CALL(packet_router, SendPacket).Times(AtLeast(1));
+  time_controller.AdvanceTime(kFixedCoalescingWindow);
+}
+
+TEST(TaskQueuePacedSenderTest, ProbingStopDuringSendLoop) {
+  // Set a low `min_probe_delta` to let probing finish during send loop.
+  ScopedKeyValueConfig trials(
+      "WebRTC-Bwe-ProbingBehavior/min_probe_delta:100us/");
+
+  GlobalSimulatedTimeController time_controller(Timestamp::Millis(1234));
+  MockPacketRouter packet_router;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Set rates so 2 packets adds 1ms of buffer level.
+  const DataSize kPacketSize = DataSize::Bytes(kDefaultPacketSize);
+  const TimeDelta kPacketPacingTime = TimeDelta::Millis(1);
+  const DataRate kPacingDataRate = 2 * kPacketSize / kPacketPacingTime;
+
+  pacer.SetPacingRates(kPacingDataRate, DataRate::Zero());
+  pacer.EnsureStarted();
+
+  EXPECT_CALL(packet_router, FetchFec).WillRepeatedly([]() {
+    return std::vector<std::unique_ptr<RtpPacketToSend>>();
+  });
+  EXPECT_CALL(packet_router, GeneratePadding(_))
+      .WillRepeatedly(
+          [](DataSize target_size) { return GeneratePadding(target_size); });
+
+  // Set probe rate.
+  const int kProbeClusterId = 1;
+  const DataRate kProbingRate = kPacingDataRate;
+
+  pacer.CreateProbeClusters(
+      {{.at_time = time_controller.GetClock()->CurrentTime(),
+        .target_data_rate = kProbingRate,
+        .target_duration = TimeDelta::Millis(15),
+        .target_probe_count = 4,
+        .id = kProbeClusterId}});
+
+  const int kPacketsToSend = 100;
+  const TimeDelta kPacketsPacedTime =
+      std::max(kPacketsToSend * kPacketSize / kPacingDataRate,
+               kPacketsToSend * kPacketSize / kProbingRate);
+
+  // Expect all packets and one padding packet sent.
+  EXPECT_CALL(packet_router, SendPacket).Times(kPacketsToSend + 1);
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kPacketsToSend));
+  time_controller.AdvanceTime(kPacketsPacedTime + TimeDelta::Millis(1));
+}
+
+TEST(TaskQueuePacedSenderTest, PostedPacketsNotSendFromRemovePacketsForSsrc) {
+  static constexpr Timestamp kStartTime = Timestamp::Millis(1234);
+  GlobalSimulatedTimeController time_controller(kStartTime);
+  ScopedKeyValueConfig trials;
+  MockPacketRouter packet_router;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  static constexpr DataRate kPacingRate =
+      DataRate::BytesPerSec(kDefaultPacketSize * 10);
+  pacer.SetPacingRates(kPacingRate, DataRate::Zero());
+  pacer.EnsureStarted();
+
+  auto encoder_queue = time_controller.GetTaskQueueFactory()->CreateTaskQueue(
+      "encoder_queue", TaskQueueFactory::Priority::HIGH);
+
+  EXPECT_CALL(packet_router, SendPacket).Times(5);
+  encoder_queue->PostTask([&pacer] {
+    pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 6));
+  });
+
+  time_controller.AdvanceTime(TimeDelta::Millis(400));
+  // 1 packet left.
+  EXPECT_EQ(pacer.OldestPacketWaitTime(), TimeDelta::Millis(400));
+  EXPECT_EQ(pacer.FirstSentPacketTime(), kStartTime);
+
+  // Enqueue packets while removing ssrcs should not send any more packets.
+  encoder_queue->PostTask(
+      [&pacer, worker_thread = time_controller.GetMainThread()] {
+        worker_thread->PostTask(
+            [&pacer] { pacer.RemovePacketsForSsrc(kVideoSsrc); });
+        pacer.EnqueuePackets(GeneratePackets(RtpPacketMediaType::kVideo, 5));
+      });
+  time_controller.AdvanceTime(TimeDelta::Seconds(1));
+  EXPECT_EQ(pacer.OldestPacketWaitTime(), TimeDelta::Zero());
+  EXPECT_EQ(pacer.FirstSentPacketTime(), kStartTime);
+  EXPECT_EQ(pacer.QueueSizeData(), DataSize::Zero());
+  EXPECT_EQ(pacer.ExpectedQueueTime(), TimeDelta::Zero());
+}
+
+TEST(TaskQueuePacedSenderTest, Stats) {
+  static constexpr Timestamp kStartTime = Timestamp::Millis(1234);
+  GlobalSimulatedTimeController time_controller(kStartTime);
+  MockPacketRouter packet_router;
+  ScopedKeyValueConfig trials;
+  TaskQueuePacedSender pacer(time_controller.GetClock(), &packet_router, trials,
+
+                             PacingController::kMinSleepTime,
+                             TaskQueuePacedSender::kNoPacketHoldback);
+
+  // Simulate ~2mbps video stream, covering one second.
+  static constexpr size_t kPacketsToSend = 200;
+  static constexpr DataRate kPacingRate =
+      DataRate::BytesPerSec(kDefaultPacketSize * kPacketsToSend);
+  pacer.SetPacingRates(kPacingRate, DataRate::Zero());
+  pacer.EnsureStarted();
+
+  // Allowed `QueueSizeData` and `ExpectedQueueTime` deviation.
+  static constexpr size_t kAllowedPacketsDeviation = 1;
+  static constexpr DataSize kAllowedQueueSizeDeviation =
+      DataSize::Bytes(kDefaultPacketSize * kAllowedPacketsDeviation);
+  static constexpr TimeDelta kAllowedQueueTimeDeviation =
+      kAllowedQueueSizeDeviation / kPacingRate;
+
+  DataSize expected_queue_size = DataSize::MinusInfinity();
+  TimeDelta expected_queue_time = TimeDelta::MinusInfinity();
+
+  EXPECT_CALL(packet_router, SendPacket).Times(kPacketsToSend);
+
+  // Stats before insert any packets.
+  EXPECT_TRUE(pacer.OldestPacketWaitTime().IsZero());
+  EXPECT_FALSE(pacer.FirstSentPacketTime().has_value());
+  EXPECT_TRUE(pacer.QueueSizeData().IsZero());
+  EXPECT_TRUE(pacer.ExpectedQueueTime().IsZero());
+
+  pacer.EnqueuePackets(
+      GeneratePackets(RtpPacketMediaType::kVideo, kPacketsToSend));
+
+  // Advance to 200ms.
+  time_controller.AdvanceTime(TimeDelta::Millis(200));
+  EXPECT_EQ(pacer.OldestPacketWaitTime(), TimeDelta::Millis(200));
+  EXPECT_EQ(pacer.FirstSentPacketTime(), kStartTime);
+
+  expected_queue_size = kPacingRate * TimeDelta::Millis(800);
+  expected_queue_time = expected_queue_size / kPacingRate;
+  EXPECT_NEAR(pacer.QueueSizeData().bytes(), expected_queue_size.bytes(),
+              kAllowedQueueSizeDeviation.bytes());
+  EXPECT_NEAR(pacer.ExpectedQueueTime().ms(), expected_queue_time.ms(),
+              kAllowedQueueTimeDeviation.ms());
+
+  // Advance to 500ms.
+  time_controller.AdvanceTime(TimeDelta::Millis(300));
+  EXPECT_EQ(pacer.OldestPacketWaitTime(), TimeDelta::Millis(500));
+  EXPECT_EQ(pacer.FirstSentPacketTime(), kStartTime);
+
+  expected_queue_size = kPacingRate * TimeDelta::Millis(500);
+  expected_queue_time = expected_queue_size / kPacingRate;
+  EXPECT_NEAR(pacer.QueueSizeData().bytes(), expected_queue_size.bytes(),
+              kAllowedQueueSizeDeviation.bytes());
+  EXPECT_NEAR(pacer.ExpectedQueueTime().ms(), expected_queue_time.ms(),
+              kAllowedQueueTimeDeviation.ms());
+
+  // Advance to 1000ms+, expect all packets to be sent.
+  time_controller.AdvanceTime(TimeDelta::Millis(500) +
+                              kAllowedQueueTimeDeviation);
+  EXPECT_TRUE(pacer.OldestPacketWaitTime().IsZero());
+  EXPECT_EQ(pacer.FirstSentPacketTime(), kStartTime);
+  EXPECT_TRUE(pacer.QueueSizeData().IsZero());
+  EXPECT_TRUE(pacer.ExpectedQueueTime().IsZero());
+}
+
+}  // namespace test
+}  // namespace webrtc