Adds Rtt tracker for PCC.

This is a part of series of CLs adding PCC (Performance-oriented
Congestion Control).

Bug: webrtc:9434
Change-Id: Idd36d8abea008623ac64b939d0de7ee6001f7f23
Reviewed-on: https://webrtc-review.googlesource.com/86951
Commit-Queue: Anastasia Koloskova <koloskova@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24364}

modules/congestion_controller/BUILD.gn

@@ -125,6 +125,7 @@ if (rtc_include_tests) {
       "bbr:bbr_unittests",
       "goog_cc:estimators",
       "goog_cc:goog_cc_unittests",
+      "pcc:pcc_unittests",
       "rtp:congestion_controller_unittests",
     ]
     if (!build_with_chromium && is_clang) {

modules/congestion_controller/pcc/BUILD.gn

@@ -0,0 +1,37 @@
+# Copyright (c) 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.
+
+import("../../../webrtc.gni")
+
+rtc_static_library("rtt_tracker") {
+  sources = [
+    "rtt_tracker.cc",
+    "rtt_tracker.h",
+  ]
+  deps = [
+    "../../../api/transport:network_control",
+    "../../../rtc_base:rtc_base_approved",
+  ]
+}
+
+if (rtc_include_tests) {
+  rtc_source_set("pcc_unittests") {
+    testonly = true
+    sources = [
+      "rtt_tracker_unittest.cc",
+    ]
+    deps = [
+      ":rtt_tracker",
+      "../../../api/transport:network_control_test",
+      "../../../api/units:data_rate",
+      "../../../api/units:time_delta",
+      "../../../api/units:timestamp",
+      "../../../test:test_support",
+    ]
+  }
+}

modules/congestion_controller/pcc/rtt_tracker.cc

@@ -0,0 +1,42 @@
+/*
+ *  Copyright (c) 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.
+ */
+
+#include <algorithm>
+
+#include "modules/congestion_controller/pcc/rtt_tracker.h"
+
+namespace webrtc {
+namespace pcc {
+
+RttTracker::RttTracker(TimeDelta initial_rtt, double alpha)
+    : rtt_estimate_(initial_rtt), alpha_(alpha) {}
+
+void RttTracker::OnPacketsFeedback(
+    const std::vector<PacketResult>& packet_feedbacks,
+    Timestamp feedback_received_time) {
+  TimeDelta packet_rtt = TimeDelta::MinusInfinity();
+  for (const PacketResult& packet_result : packet_feedbacks) {
+    if (!packet_result.sent_packet.has_value() ||
+        packet_result.receive_time.IsInfinite())
+      continue;
+    packet_rtt = std::max<TimeDelta>(
+        packet_rtt,
+        feedback_received_time - packet_result.sent_packet->send_time);
+  }
+  if (packet_rtt.IsFinite())
+    rtt_estimate_ = (1 - alpha_) * rtt_estimate_ + alpha_ * packet_rtt;
+}
+
+TimeDelta RttTracker::GetRtt() const {
+  return rtt_estimate_;
+}
+
+}  // namespace pcc
+}  // namespace webrtc

modules/congestion_controller/pcc/rtt_tracker.h

@@ -0,0 +1,38 @@
+/*
+ *  Copyright (c) 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.
+ */
+
+#ifndef MODULES_CONGESTION_CONTROLLER_PCC_RTT_TRACKER_H_
+#define MODULES_CONGESTION_CONTROLLER_PCC_RTT_TRACKER_H_
+
+#include <vector>
+
+#include "api/transport/network_control.h"
+#include "api/transport/network_types.h"
+
+namespace webrtc {
+namespace pcc {
+
+class RttTracker {
+ public:
+  RttTracker(TimeDelta initial_rtt, double alpha);
+  // Updates RTT estimate.
+  void OnPacketsFeedback(const std::vector<PacketResult>& packet_feedbacks,
+                         Timestamp feedback_received_time);
+  TimeDelta GetRtt() const;
+
+ private:
+  TimeDelta rtt_estimate_;
+  double alpha_;
+};
+
+}  // namespace pcc
+}  // namespace webrtc
+
+#endif  // MODULES_CONGESTION_CONTROLLER_PCC_RTT_TRACKER_H_

modules/congestion_controller/pcc/rtt_tracker_unittest.cc

@@ -0,0 +1,70 @@
+/*
+ *  Copyright (c) 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.
+ */
+
+#include "modules/congestion_controller/pcc/rtt_tracker.h"
+#include "test/gtest.h"
+
+namespace webrtc {
+namespace pcc {
+namespace test {
+namespace {
+const TimeDelta kInitialRtt = TimeDelta::us(10);
+constexpr double kAlpha = 0.9;
+const Timestamp kStartTime = Timestamp::seconds(0);
+
+PacketResult GetPacketWithRtt(TimeDelta rtt) {
+  SentPacket packet;
+  packet.send_time = kStartTime;
+  PacketResult packet_result;
+  packet_result.sent_packet = packet;
+  if (rtt.IsFinite()) {
+    packet_result.receive_time = kStartTime + rtt;
+  } else {
+    packet_result.receive_time = Timestamp::Infinity();
+  }
+  return packet_result;
+}
+}  // namespace
+
+TEST(PccRttTrackerTest, InitialValue) {
+  RttTracker tracker{kInitialRtt, kAlpha};
+  EXPECT_EQ(kInitialRtt, tracker.GetRtt());
+  for (int i = 0; i < 100; ++i) {
+    tracker.OnPacketsFeedback({GetPacketWithRtt(kInitialRtt)},
+                              kStartTime + kInitialRtt);
+  }
+  EXPECT_EQ(kInitialRtt, tracker.GetRtt());
+}
+
+TEST(PccRttTrackerTest, DoNothingWhenPacketIsLost) {
+  RttTracker tracker{kInitialRtt, kAlpha};
+  tracker.OnPacketsFeedback({GetPacketWithRtt(TimeDelta::PlusInfinity())},
+                            kStartTime + kInitialRtt);
+  EXPECT_EQ(tracker.GetRtt(), kInitialRtt);
+}
+
+TEST(PccRttTrackerTest, ChangeInRtt) {
+  RttTracker tracker{kInitialRtt, kAlpha};
+  const TimeDelta kNewRtt = TimeDelta::us(100);
+  tracker.OnPacketsFeedback({GetPacketWithRtt(kNewRtt)}, kStartTime + kNewRtt);
+  EXPECT_GT(tracker.GetRtt(), kInitialRtt);
+  EXPECT_LE(tracker.GetRtt(), kNewRtt);
+  for (int i = 0; i < 100; ++i) {
+    tracker.OnPacketsFeedback({GetPacketWithRtt(kNewRtt)},
+                              kStartTime + kNewRtt);
+  }
+  const TimeDelta absolute_error = TimeDelta::us(1);
+  EXPECT_NEAR(tracker.GetRtt().us(), kNewRtt.us(), absolute_error.us());
+  EXPECT_LE(tracker.GetRtt(), kNewRtt);
+}
+
+}  // namespace test
+}  // namespace pcc
+}  // namespace webrtc