Added implementation of three classes in BBR,with unit-tests.

webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.cc

+/*
+ *  Copyright (c) 2017 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 "webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.h"
+
+#include <algorithm>
+
+#include "webrtc/modules/remote_bitrate_estimator/test/estimators/bbr.h"
+
+namespace webrtc {
+namespace testing {
+namespace bwe {
+namespace {
+// kStartingCongestionWindow is used to set congestion window when bandwidth
+// delay product is equal to zero, so that we don't set window to zero as well.
+// Chosen randomly by me, because this value shouldn't make any significant
+// difference, as bandwidth delay product is more than zero almost every time.
+const int kStartingCongestionWindow = 6000;

[terelius, 2017/07/12 10:34:50]

Is there any reason for not setting it to the minimum possible, i.e. reuse
kMinimumCongestionWindow?

[gnish1, 2017/07/12 12:23:18]

It depends on the situation, this variable shouldn't be used at all in most
cases, it is just for safety, but it is different from kMinimumCongestionWindow,
its value may change in the upcoming cl's but for now I don't see any clear
reason to do so, it just exists because it is different from
kMinimumCongestionWindow.

+// Size of congestion window while in PROBE_RTT mode, suggested by BBR's source
+// code of QUIC's implementation.
+const int kMinimumCongestionWindow = 5840;
+}  // namespace
+
+CongestionWindow::CongestionWindow() : data_inflight_(0) {}
+
+CongestionWindow::~CongestionWindow() {}
+
+int CongestionWindow::GetCongestionWindow(BbrBweSender::Mode mode,
+                                          int64_t bandwidth_estimate,
+                                          int64_t min_rtt,
+                                          float gain,
+                                          size_t bytes_acked,
+                                          int multiplier) {
+  if (mode == BbrBweSender::PROBE_RTT)
+    return kMinimumCongestionWindow;
+  return GetTargetCongestionWindow(bandwidth_estimate, min_rtt, gain) +
+         multiplier * bytes_acked;

[terelius, 2017/07/12 10:34:51]

Why do we add bytes_acked?

[gnish1, 2017/07/12 12:23:18]

Done.

+}
+
+void CongestionWindow::PacketSent(size_t sent_packet_size) {
+  data_inflight_ += sent_packet_size;
+}
+
+void CongestionWindow::AckReceived(size_t received_packet_size) {
+  data_inflight_ -= received_packet_size;
+}
+
+int CongestionWindow::GetTargetCongestionWindow(int64_t bandwidth_estimate,
+                                                int64_t min_rtt,
+                                                float gain) {
+  int bdp = min_rtt * bandwidth_estimate;
+  int congestion_window = bdp * gain;
+  // Congestion window could be zero in rare cases, when either no bandwidth
+  // estimate is available, or path's min_rtt value is zero.
+  if (!congestion_window)
+    congestion_window = gain * kStartingCongestionWindow;

[terelius, 2017/07/12 10:34:50]

Shouldn't the gain factor already be part of kStartingCongestionWindow?

[gnish1, 2017/07/12 12:23:18]

No, gain factor may change, so it is not possible to include it in
kStartingCongestionWindow.

+  return std::max(congestion_window, kMinimumCongestionWindow);
+}
+}  // namespace bwe
+}  // namespace testing
+}  // namespace webrtc