Refactor of QUIC's rtt storage and calculation to have a single RttStats

net/quic/congestion_control/rtt_stats.cc

Index: net/quic/congestion_control/rtt_stats.cc
diff --git a/net/quic/congestion_control/rtt_stats.cc b/net/quic/congestion_control/rtt_stats.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8f9e1d1910ac9dd77ee319dbe47db6ab6ff692d2
--- /dev/null
+++ b/net/quic/congestion_control/rtt_stats.cc
@@ -0,0 +1,81 @@
+// Copyright (c) 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "net/quic/congestion_control/rtt_stats.h"
+
+namespace net {
+
+namespace {
+
+// Default initial rtt used before any samples are received.
+const int kInitialRttMs = 100;
+const float kAlpha = 0.125f;
+const float kOneMinusAlpha = (1 - kAlpha);
+const float kBeta = 0.25f;
+const float kOneMinusBeta = (1 - kBeta);
+
+}
+
+RttStats::RttStats()
+    : latest_rtt_(QuicTime::Delta::Zero()),
+      min_rtt_(QuicTime::Delta::Zero()),
+      smoothed_rtt_(QuicTime::Delta::Zero()),
+      mean_deviation_(QuicTime::Delta::Zero()),
+      initial_rtt_us_(kInitialRttMs * base::Time::kMicrosecondsPerMillisecond) {
+}
+
+bool RttStats::HasUpdates() const {
+  return !smoothed_rtt_.IsZero();
+}
+
+// Updates the RTT based on a new sample.
+void RttStats::UpdateRtt(QuicTime::Delta send_delta,
+                         QuicTime::Delta ack_delay) {
+  QuicTime::Delta rtt_sample(QuicTime::Delta::Zero());
+  if (send_delta > ack_delay) {
+    rtt_sample = send_delta.Subtract(ack_delay);
+  } else if (!HasUpdates()) {
+    // Even though we received information from the peer suggesting
+    // an invalid (negative) RTT, we can use the send delta as an
+    // approximation until we get a better estimate.
+    rtt_sample = send_delta;
+  }
+
+  if (rtt_sample.IsInfinite() || rtt_sample.IsZero()) {
+    DVLOG(1) << "Ignoring rtt, because it's "
+             << (rtt_sample.IsZero() ? "Zero" : "Infinite");
+    return;
+  }
+  // RTT can't be negative.
+  DCHECK_LT(0, rtt_sample.ToMicroseconds());
+
+  latest_rtt_ = rtt_sample;
+  // First time call or link delay decreases.
+  if (min_rtt_.IsZero() || min_rtt_ > rtt_sample) {
+    min_rtt_ = rtt_sample;
+  }
+  // First time call.
+  if (!HasUpdates()) {
+    smoothed_rtt_ = rtt_sample;
+    mean_deviation_ = QuicTime::Delta::FromMicroseconds(
+        rtt_sample.ToMicroseconds() / 2);
+  } else {
+    mean_deviation_ = QuicTime::Delta::FromMicroseconds(
+        kOneMinusBeta * mean_deviation_.ToMicroseconds() +
+        kBeta * std::abs(smoothed_rtt_.Subtract(rtt_sample).ToMicroseconds()));
+    smoothed_rtt_ = smoothed_rtt_.Multiply(kOneMinusAlpha).Add(
+        rtt_sample.Multiply(kAlpha));
+    DVLOG(1) << "Cubic; smoothed_rtt(us):" << smoothed_rtt_.ToMicroseconds()
+             << " mean_deviation(us):" << mean_deviation_.ToMicroseconds();
+  }
+}
+
+QuicTime::Delta RttStats::SmoothedRtt() const {
+  if (!HasUpdates()) {
+    return QuicTime::Delta::FromMicroseconds(initial_rtt_us_);
+  }
+  return smoothed_rtt_;
+}
+
+}  // namespace net