modules/congestion_controller/bbr/bandwidth_sampler_unittest.cc - src - Git at Google

 /*
  *  Copyright 2018 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.
  */
 // Based on the Quic implementation in Chromium.

 #include "modules/congestion_controller/bbr/bandwidth_sampler.h"

 #include <stddef.h>

 #include "test/gtest.h"

 namespace webrtc {
 namespace bbr {
 namespace test {

 class BandwidthSamplerPeer {
  public:
   static size_t GetNumberOfTrackedPackets(const BandwidthSampler& sampler) {
     return sampler.connection_state_map_.number_of_present_entries();
   }

   static DataSize GetPacketSize(const BandwidthSampler& sampler,
                                 int64_t packet_number) {
     return sampler.connection_state_map_.GetEntry(packet_number)->size;
   }
 };

 const int64_t kRegularPacketSizeBytes = 1280;
 // Enforce divisibility for some of the tests.
 static_assert((kRegularPacketSizeBytes & 31) == 0,
               "kRegularPacketSizeBytes has to be five times divisible by 2");

 const DataSize kRegularPacketSize = DataSize::bytes(kRegularPacketSizeBytes);

 // A test fixture with utility methods for BandwidthSampler tests.
 class BandwidthSamplerTest : public ::testing::Test {
  protected:
   BandwidthSamplerTest()
       : clock_(Timestamp::Seconds(100)), bytes_in_flight_(DataSize::Zero()) {}

   Timestamp clock_;
   BandwidthSampler sampler_;
   DataSize bytes_in_flight_;

   void SendPacketInner(int64_t packet_number, DataSize bytes) {
     sampler_.OnPacketSent(clock_, packet_number, bytes, bytes_in_flight_);
     bytes_in_flight_ += bytes;
   }

   void SendPacket(int64_t packet_number) {
     SendPacketInner(packet_number, kRegularPacketSize);
   }

   BandwidthSample AckPacketInner(int64_t packet_number) {
     DataSize size =
         BandwidthSamplerPeer::GetPacketSize(sampler_, packet_number);
     bytes_in_flight_ -= size;
     return sampler_.OnPacketAcknowledged(clock_, packet_number);
   }

   // Acknowledge receipt of a packet and expect it to be not app-limited.
   DataRate AckPacket(int64_t packet_number) {
     BandwidthSample sample = AckPacketInner(packet_number);
     EXPECT_FALSE(sample.is_app_limited);
     return sample.bandwidth;
   }

   void LosePacket(int64_t packet_number) {
     DataSize size =
         BandwidthSamplerPeer::GetPacketSize(sampler_, packet_number);
     bytes_in_flight_ -= size;
     sampler_.OnPacketLost(packet_number);
   }

   // Sends one packet and acks it.  Then, send 20 packets.  Finally, send
   // another 20 packets while acknowledging previous 20.
   void Send40PacketsAndAckFirst20(TimeDelta time_between_packets) {
     // Send 20 packets at a constant inter-packet time.
     for (int64_t i = 1; i <= 20; i++) {
       SendPacket(i);
       clock_ += time_between_packets;
     }

     // Ack packets 1 to 20, while sending new packets at the same rate as
     // before.
     for (int64_t i = 1; i <= 20; i++) {
       AckPacket(i);
       SendPacket(i + 20);
       clock_ += time_between_packets;
     }
   }
 };

 // Test the sampler in a simple stop-and-wait sender setting.
 TEST_F(BandwidthSamplerTest, SendAndWait) {
   TimeDelta time_between_packets = TimeDelta::Millis(10);
   DataRate expected_bandwidth =
       kRegularPacketSize * 100 / TimeDelta::Seconds(1);

   // Send packets at the constant bandwidth.
   for (int64_t i = 1; i < 20; i++) {
     SendPacket(i);
     clock_ += time_between_packets;
     DataRate current_sample = AckPacket(i);
     EXPECT_EQ(expected_bandwidth, current_sample);
   }

   // Send packets at the exponentially decreasing bandwidth.
   for (int64_t i = 20; i < 25; i++) {
     time_between_packets = time_between_packets * 2;
     expected_bandwidth = expected_bandwidth * 0.5;

     SendPacket(i);
     clock_ += time_between_packets;
     DataRate current_sample = AckPacket(i);
     EXPECT_EQ(expected_bandwidth, current_sample);
   }
   EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   EXPECT_TRUE(bytes_in_flight_.IsZero());
 }

 // Test the sampler during regular windowed sender scenario with fixed
 // CWND of 20.
 TEST_F(BandwidthSamplerTest, SendPaced) {
   const TimeDelta time_between_packets = TimeDelta::Millis(1);
   DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

   Send40PacketsAndAckFirst20(time_between_packets);

   // Ack the packets 21 to 40, arriving at the correct bandwidth.
   DataRate last_bandwidth = DataRate::Zero();
   for (int64_t i = 21; i <= 40; i++) {
     last_bandwidth = AckPacket(i);
     EXPECT_EQ(expected_bandwidth, last_bandwidth);
     clock_ += time_between_packets;
   }
   EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   EXPECT_TRUE(bytes_in_flight_.IsZero());
 }

 // Test the sampler in a scenario where 50% of packets is consistently lost.
 TEST_F(BandwidthSamplerTest, SendWithLosses) {
   const TimeDelta time_between_packets = TimeDelta::Millis(1);
   DataRate expected_bandwidth = kRegularPacketSize / time_between_packets * 0.5;

   // Send 20 packets, each 1 ms apart.
   for (int64_t i = 1; i <= 20; i++) {
     SendPacket(i);
     clock_ += time_between_packets;
   }

   // Ack packets 1 to 20, losing every even-numbered packet, while sending new
   // packets at the same rate as before.
   for (int64_t i = 1; i <= 20; i++) {
     if (i % 2 == 0) {
       AckPacket(i);
     } else {
       LosePacket(i);
     }
     SendPacket(i + 20);
     clock_ += time_between_packets;
   }

   // Ack the packets 21 to 40 with the same loss pattern.
   DataRate last_bandwidth = DataRate::Zero();
   for (int64_t i = 21; i <= 40; i++) {
     if (i % 2 == 0) {
       last_bandwidth = AckPacket(i);
       EXPECT_EQ(expected_bandwidth, last_bandwidth);
     } else {
       LosePacket(i);
     }
     clock_ += time_between_packets;
   }
   EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   EXPECT_TRUE(bytes_in_flight_.IsZero());
 }

 // Simulate a situation where ACKs arrive in burst and earlier than usual, thus
 // producing an ACK rate which is higher than the original send rate.
 TEST_F(BandwidthSamplerTest, CompressedAck) {
   const TimeDelta time_between_packets = TimeDelta::Millis(1);
   DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

   Send40PacketsAndAckFirst20(time_between_packets);

   // Simulate an RTT somewhat lower than the one for 1-to-21 transmission.
   clock_ += time_between_packets * 15;

   // Ack the packets 21 to 40 almost immediately at once.
   DataRate last_bandwidth = DataRate::Zero();
   TimeDelta ridiculously_small_time_delta = TimeDelta::Micros(20);
   for (int64_t i = 21; i <= 40; i++) {
     last_bandwidth = AckPacket(i);
     clock_ += ridiculously_small_time_delta;
   }
   EXPECT_EQ(expected_bandwidth, last_bandwidth);
   EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   EXPECT_TRUE(bytes_in_flight_.IsZero());
 }

 // Tests receiving ACK packets in the reverse order.
 TEST_F(BandwidthSamplerTest, ReorderedAck) {
   const TimeDelta time_between_packets = TimeDelta::Millis(1);
   DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

   Send40PacketsAndAckFirst20(time_between_packets);

   // Ack the packets 21 to 40 in the reverse order, while sending packets 41 to
   // 60.
   DataRate last_bandwidth = DataRate::Zero();
   for (int64_t i = 0; i < 20; i++) {
     last_bandwidth = AckPacket(40 - i);
     EXPECT_EQ(expected_bandwidth, last_bandwidth);
     SendPacket(41 + i);
     clock_ += time_between_packets;
   }

   // Ack the packets 41 to 60, now in the regular order.
   for (int64_t i = 41; i <= 60; i++) {
     last_bandwidth = AckPacket(i);
     EXPECT_EQ(expected_bandwidth, last_bandwidth);
     clock_ += time_between_packets;
   }
   EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   EXPECT_TRUE(bytes_in_flight_.IsZero());
 }

 // Test the app-limited logic.
 TEST_F(BandwidthSamplerTest, AppLimited) {
   const TimeDelta time_between_packets = TimeDelta::Millis(1);
   DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

   Send40PacketsAndAckFirst20(time_between_packets);

   // We are now app-limited. Ack 21 to 40 as usual, but do not send anything for
   // now.
   sampler_.OnAppLimited();
   for (int64_t i = 21; i <= 40; i++) {
     DataRate current_sample = AckPacket(i);
     EXPECT_EQ(expected_bandwidth, current_sample);
     clock_ += time_between_packets;
   }

   // Enter quiescence.
   clock_ += TimeDelta::Seconds(1);

   // Send packets 41 to 60, all of which would be marked as app-limited.
   for (int64_t i = 41; i <= 60; i++) {
     SendPacket(i);
     clock_ += time_between_packets;
   }

   // Ack packets 41 to 60, while sending packets 61 to 80.  41 to 60 should be
   // app-limited and underestimate the bandwidth due to that.
   for (int64_t i = 41; i <= 60; i++) {
     BandwidthSample sample = AckPacketInner(i);
     EXPECT_TRUE(sample.is_app_limited);
     EXPECT_LT(sample.bandwidth, 0.7f * expected_bandwidth);

     SendPacket(i + 20);
     clock_ += time_between_packets;
   }

   // Run out of packets, and then ack packet 61 to 80, all of which should have
   // correct non-app-limited samples.
   for (int64_t i = 61; i <= 80; i++) {
     DataRate last_bandwidth = AckPacket(i);
     EXPECT_EQ(expected_bandwidth, last_bandwidth);
     clock_ += time_between_packets;
   }

   EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   EXPECT_TRUE(bytes_in_flight_.IsZero());
 }

 // Test the samples taken at the first flight of packets sent.
 TEST_F(BandwidthSamplerTest, FirstRoundTrip) {
   const TimeDelta time_between_packets = TimeDelta::Millis(1);
   const TimeDelta rtt = TimeDelta::Millis(800);
   const int num_packets = 10;
   const DataSize num_bytes = kRegularPacketSize * num_packets;
   const DataRate real_bandwidth = num_bytes / rtt;

   for (int64_t i = 1; i <= 10; i++) {
     SendPacket(i);
     clock_ += time_between_packets;
   }

   clock_ += rtt - num_packets * time_between_packets;

   DataRate last_sample = DataRate::Zero();
   for (int64_t i = 1; i <= 10; i++) {
     DataRate sample = AckPacket(i);
     EXPECT_GT(sample, last_sample);
     last_sample = sample;
     clock_ += time_between_packets;
   }

   // The final measured sample for the first flight of sample is expected to be
   // smaller than the real bandwidth, yet it should not lose more than 10%. The
   // specific value of the error depends on the difference between the RTT and
   // the time it takes to exhaust the congestion window (i.e. in the limit when
   // all packets are sent simultaneously, last sample would indicate the real
   // bandwidth).
   EXPECT_LT(last_sample, real_bandwidth);
   EXPECT_GT(last_sample, 0.9f * real_bandwidth);
 }

 // Test sampler's ability to remove obsolete packets.
 TEST_F(BandwidthSamplerTest, RemoveObsoletePackets) {
   SendPacket(1);
   SendPacket(2);
   SendPacket(3);
   SendPacket(4);
   SendPacket(5);

   clock_ += TimeDelta::Millis(100);

   EXPECT_EQ(5u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   sampler_.RemoveObsoletePackets(4);
   EXPECT_EQ(2u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   sampler_.OnPacketLost(4);
   EXPECT_EQ(1u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
   AckPacket(5);
   EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
 }

 }  // namespace test
 }  // namespace bbr
 }  // namespace webrtc
	/*
	* Copyright 2018 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.
	*/
	// Based on the Quic implementation in Chromium.

	#include "modules/congestion_controller/bbr/bandwidth_sampler.h"

	#include <stddef.h>

	#include "test/gtest.h"

	namespace webrtc {
	namespace bbr {
	namespace test {

	class BandwidthSamplerPeer {
	public:
	static size_t GetNumberOfTrackedPackets(const BandwidthSampler& sampler) {
	return sampler.connection_state_map_.number_of_present_entries();
	}

	static DataSize GetPacketSize(const BandwidthSampler& sampler,
	int64_t packet_number) {
	return sampler.connection_state_map_.GetEntry(packet_number)->size;
	}
	};

	const int64_t kRegularPacketSizeBytes = 1280;
	// Enforce divisibility for some of the tests.
	static_assert((kRegularPacketSizeBytes & 31) == 0,
	"kRegularPacketSizeBytes has to be five times divisible by 2");

	const DataSize kRegularPacketSize = DataSize::bytes(kRegularPacketSizeBytes);

	// A test fixture with utility methods for BandwidthSampler tests.
	class BandwidthSamplerTest : public ::testing::Test {
	protected:
	BandwidthSamplerTest()
	: clock_(Timestamp::Seconds(100)), bytes_in_flight_(DataSize::Zero()) {}

	Timestamp clock_;
	BandwidthSampler sampler_;
	DataSize bytes_in_flight_;

	void SendPacketInner(int64_t packet_number, DataSize bytes) {
	sampler_.OnPacketSent(clock_, packet_number, bytes, bytes_in_flight_);
	bytes_in_flight_ += bytes;
	}

	void SendPacket(int64_t packet_number) {
	SendPacketInner(packet_number, kRegularPacketSize);
	}

	BandwidthSample AckPacketInner(int64_t packet_number) {
	DataSize size =
	BandwidthSamplerPeer::GetPacketSize(sampler_, packet_number);
	bytes_in_flight_ -= size;
	return sampler_.OnPacketAcknowledged(clock_, packet_number);
	}

	// Acknowledge receipt of a packet and expect it to be not app-limited.
	DataRate AckPacket(int64_t packet_number) {
	BandwidthSample sample = AckPacketInner(packet_number);
	EXPECT_FALSE(sample.is_app_limited);
	return sample.bandwidth;
	}

	void LosePacket(int64_t packet_number) {
	DataSize size =
	BandwidthSamplerPeer::GetPacketSize(sampler_, packet_number);
	bytes_in_flight_ -= size;
	sampler_.OnPacketLost(packet_number);
	}

	// Sends one packet and acks it. Then, send 20 packets. Finally, send
	// another 20 packets while acknowledging previous 20.
	void Send40PacketsAndAckFirst20(TimeDelta time_between_packets) {
	// Send 20 packets at a constant inter-packet time.
	for (int64_t i = 1; i <= 20; i++) {
	SendPacket(i);
	clock_ += time_between_packets;
	}

	// Ack packets 1 to 20, while sending new packets at the same rate as
	// before.
	for (int64_t i = 1; i <= 20; i++) {
	AckPacket(i);
	SendPacket(i + 20);
	clock_ += time_between_packets;
	}
	}
	};

	// Test the sampler in a simple stop-and-wait sender setting.
	TEST_F(BandwidthSamplerTest, SendAndWait) {
	TimeDelta time_between_packets = TimeDelta::Millis(10);
	DataRate expected_bandwidth =
	kRegularPacketSize * 100 / TimeDelta::Seconds(1);

	// Send packets at the constant bandwidth.
	for (int64_t i = 1; i < 20; i++) {
	SendPacket(i);
	clock_ += time_between_packets;
	DataRate current_sample = AckPacket(i);
	EXPECT_EQ(expected_bandwidth, current_sample);
	}

	// Send packets at the exponentially decreasing bandwidth.
	for (int64_t i = 20; i < 25; i++) {
	time_between_packets = time_between_packets * 2;
	expected_bandwidth = expected_bandwidth * 0.5;

	SendPacket(i);
	clock_ += time_between_packets;
	DataRate current_sample = AckPacket(i);
	EXPECT_EQ(expected_bandwidth, current_sample);
	}
	EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	EXPECT_TRUE(bytes_in_flight_.IsZero());
	}

	// Test the sampler during regular windowed sender scenario with fixed
	// CWND of 20.
	TEST_F(BandwidthSamplerTest, SendPaced) {
	const TimeDelta time_between_packets = TimeDelta::Millis(1);
	DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

	Send40PacketsAndAckFirst20(time_between_packets);

	// Ack the packets 21 to 40, arriving at the correct bandwidth.
	DataRate last_bandwidth = DataRate::Zero();
	for (int64_t i = 21; i <= 40; i++) {
	last_bandwidth = AckPacket(i);
	EXPECT_EQ(expected_bandwidth, last_bandwidth);
	clock_ += time_between_packets;
	}
	EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	EXPECT_TRUE(bytes_in_flight_.IsZero());
	}

	// Test the sampler in a scenario where 50% of packets is consistently lost.
	TEST_F(BandwidthSamplerTest, SendWithLosses) {
	const TimeDelta time_between_packets = TimeDelta::Millis(1);
	DataRate expected_bandwidth = kRegularPacketSize / time_between_packets * 0.5;

	// Send 20 packets, each 1 ms apart.
	for (int64_t i = 1; i <= 20; i++) {
	SendPacket(i);
	clock_ += time_between_packets;
	}

	// Ack packets 1 to 20, losing every even-numbered packet, while sending new
	// packets at the same rate as before.
	for (int64_t i = 1; i <= 20; i++) {
	if (i % 2 == 0) {
	AckPacket(i);
	} else {
	LosePacket(i);
	}
	SendPacket(i + 20);
	clock_ += time_between_packets;
	}

	// Ack the packets 21 to 40 with the same loss pattern.
	DataRate last_bandwidth = DataRate::Zero();
	for (int64_t i = 21; i <= 40; i++) {
	if (i % 2 == 0) {
	last_bandwidth = AckPacket(i);
	EXPECT_EQ(expected_bandwidth, last_bandwidth);
	} else {
	LosePacket(i);
	}
	clock_ += time_between_packets;
	}
	EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	EXPECT_TRUE(bytes_in_flight_.IsZero());
	}

	// Simulate a situation where ACKs arrive in burst and earlier than usual, thus
	// producing an ACK rate which is higher than the original send rate.
	TEST_F(BandwidthSamplerTest, CompressedAck) {
	const TimeDelta time_between_packets = TimeDelta::Millis(1);
	DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

	Send40PacketsAndAckFirst20(time_between_packets);

	// Simulate an RTT somewhat lower than the one for 1-to-21 transmission.
	clock_ += time_between_packets * 15;

	// Ack the packets 21 to 40 almost immediately at once.
	DataRate last_bandwidth = DataRate::Zero();
	TimeDelta ridiculously_small_time_delta = TimeDelta::Micros(20);
	for (int64_t i = 21; i <= 40; i++) {
	last_bandwidth = AckPacket(i);
	clock_ += ridiculously_small_time_delta;
	}
	EXPECT_EQ(expected_bandwidth, last_bandwidth);
	EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	EXPECT_TRUE(bytes_in_flight_.IsZero());
	}

	// Tests receiving ACK packets in the reverse order.
	TEST_F(BandwidthSamplerTest, ReorderedAck) {
	const TimeDelta time_between_packets = TimeDelta::Millis(1);
	DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

	Send40PacketsAndAckFirst20(time_between_packets);

	// Ack the packets 21 to 40 in the reverse order, while sending packets 41 to
	// 60.
	DataRate last_bandwidth = DataRate::Zero();
	for (int64_t i = 0; i < 20; i++) {
	last_bandwidth = AckPacket(40 - i);
	EXPECT_EQ(expected_bandwidth, last_bandwidth);
	SendPacket(41 + i);
	clock_ += time_between_packets;
	}

	// Ack the packets 41 to 60, now in the regular order.
	for (int64_t i = 41; i <= 60; i++) {
	last_bandwidth = AckPacket(i);
	EXPECT_EQ(expected_bandwidth, last_bandwidth);
	clock_ += time_between_packets;
	}
	EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	EXPECT_TRUE(bytes_in_flight_.IsZero());
	}

	// Test the app-limited logic.
	TEST_F(BandwidthSamplerTest, AppLimited) {
	const TimeDelta time_between_packets = TimeDelta::Millis(1);
	DataRate expected_bandwidth = kRegularPacketSize / time_between_packets;

	Send40PacketsAndAckFirst20(time_between_packets);

	// We are now app-limited. Ack 21 to 40 as usual, but do not send anything for
	// now.
	sampler_.OnAppLimited();
	for (int64_t i = 21; i <= 40; i++) {
	DataRate current_sample = AckPacket(i);
	EXPECT_EQ(expected_bandwidth, current_sample);
	clock_ += time_between_packets;
	}

	// Enter quiescence.
	clock_ += TimeDelta::Seconds(1);

	// Send packets 41 to 60, all of which would be marked as app-limited.
	for (int64_t i = 41; i <= 60; i++) {
	SendPacket(i);
	clock_ += time_between_packets;
	}

	// Ack packets 41 to 60, while sending packets 61 to 80. 41 to 60 should be
	// app-limited and underestimate the bandwidth due to that.
	for (int64_t i = 41; i <= 60; i++) {
	BandwidthSample sample = AckPacketInner(i);
	EXPECT_TRUE(sample.is_app_limited);
	EXPECT_LT(sample.bandwidth, 0.7f * expected_bandwidth);

	SendPacket(i + 20);
	clock_ += time_between_packets;
	}

	// Run out of packets, and then ack packet 61 to 80, all of which should have
	// correct non-app-limited samples.
	for (int64_t i = 61; i <= 80; i++) {
	DataRate last_bandwidth = AckPacket(i);
	EXPECT_EQ(expected_bandwidth, last_bandwidth);
	clock_ += time_between_packets;
	}

	EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	EXPECT_TRUE(bytes_in_flight_.IsZero());
	}

	// Test the samples taken at the first flight of packets sent.
	TEST_F(BandwidthSamplerTest, FirstRoundTrip) {
	const TimeDelta time_between_packets = TimeDelta::Millis(1);
	const TimeDelta rtt = TimeDelta::Millis(800);
	const int num_packets = 10;
	const DataSize num_bytes = kRegularPacketSize * num_packets;
	const DataRate real_bandwidth = num_bytes / rtt;

	for (int64_t i = 1; i <= 10; i++) {
	SendPacket(i);
	clock_ += time_between_packets;
	}

	clock_ += rtt - num_packets * time_between_packets;

	DataRate last_sample = DataRate::Zero();
	for (int64_t i = 1; i <= 10; i++) {
	DataRate sample = AckPacket(i);
	EXPECT_GT(sample, last_sample);
	last_sample = sample;
	clock_ += time_between_packets;
	}

	// The final measured sample for the first flight of sample is expected to be
	// smaller than the real bandwidth, yet it should not lose more than 10%. The
	// specific value of the error depends on the difference between the RTT and
	// the time it takes to exhaust the congestion window (i.e. in the limit when
	// all packets are sent simultaneously, last sample would indicate the real
	// bandwidth).
	EXPECT_LT(last_sample, real_bandwidth);
	EXPECT_GT(last_sample, 0.9f * real_bandwidth);
	}

	// Test sampler's ability to remove obsolete packets.
	TEST_F(BandwidthSamplerTest, RemoveObsoletePackets) {
	SendPacket(1);
	SendPacket(2);
	SendPacket(3);
	SendPacket(4);
	SendPacket(5);

	clock_ += TimeDelta::Millis(100);

	EXPECT_EQ(5u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	sampler_.RemoveObsoletePackets(4);
	EXPECT_EQ(2u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	sampler_.OnPacketLost(4);
	EXPECT_EQ(1u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	AckPacket(5);
	EXPECT_EQ(0u, BandwidthSamplerPeer::GetNumberOfTrackedPackets(sampler_));
	}

	} // namespace test
	} // namespace bbr
	} // namespace webrtc