Project import generated by Copybara.
PiperOrigin-RevId: 237361882
Change-Id: I109a68f44db867b20f8c6a7732b0ce657133e52a
diff --git a/quic/core/congestion_control/hybrid_slow_start.cc b/quic/core/congestion_control/hybrid_slow_start.cc
new file mode 100644
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+++ b/quic/core/congestion_control/hybrid_slow_start.cc
@@ -0,0 +1,104 @@
+// Copyright (c) 2012 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/third_party/quiche/src/quic/core/congestion_control/hybrid_slow_start.h"
+
+#include <algorithm>
+
+#include "net/third_party/quiche/src/quic/platform/api/quic_logging.h"
+
+namespace quic {
+
+// Note(pwestin): the magic clamping numbers come from the original code in
+// tcp_cubic.c.
+const int64_t kHybridStartLowWindow = 16;
+// Number of delay samples for detecting the increase of delay.
+const uint32_t kHybridStartMinSamples = 8;
+// Exit slow start if the min rtt has increased by more than 1/8th.
+const int kHybridStartDelayFactorExp = 3; // 2^3 = 8
+// The original paper specifies 2 and 8ms, but those have changed over time.
+const int64_t kHybridStartDelayMinThresholdUs = 4000;
+const int64_t kHybridStartDelayMaxThresholdUs = 16000;
+
+HybridSlowStart::HybridSlowStart()
+ : started_(false),
+ hystart_found_(NOT_FOUND),
+ rtt_sample_count_(0),
+ current_min_rtt_(QuicTime::Delta::Zero()) {}
+
+void HybridSlowStart::OnPacketAcked(QuicPacketNumber acked_packet_number) {
+ // OnPacketAcked gets invoked after ShouldExitSlowStart, so it's best to end
+ // the round when the final packet of the burst is received and start it on
+ // the next incoming ack.
+ if (IsEndOfRound(acked_packet_number)) {
+ started_ = false;
+ }
+}
+
+void HybridSlowStart::OnPacketSent(QuicPacketNumber packet_number) {
+ last_sent_packet_number_ = packet_number;
+}
+
+void HybridSlowStart::Restart() {
+ started_ = false;
+ hystart_found_ = NOT_FOUND;
+}
+
+void HybridSlowStart::StartReceiveRound(QuicPacketNumber last_sent) {
+ QUIC_DVLOG(1) << "Reset hybrid slow start @" << last_sent;
+ end_packet_number_ = last_sent;
+ current_min_rtt_ = QuicTime::Delta::Zero();
+ rtt_sample_count_ = 0;
+ started_ = true;
+}
+
+bool HybridSlowStart::IsEndOfRound(QuicPacketNumber ack) const {
+ return !end_packet_number_.IsInitialized() || end_packet_number_ <= ack;
+}
+
+bool HybridSlowStart::ShouldExitSlowStart(QuicTime::Delta latest_rtt,
+ QuicTime::Delta min_rtt,
+ QuicPacketCount congestion_window) {
+ if (!started_) {
+ // Time to start the hybrid slow start.
+ StartReceiveRound(last_sent_packet_number_);
+ }
+ if (hystart_found_ != NOT_FOUND) {
+ return true;
+ }
+ // Second detection parameter - delay increase detection.
+ // Compare the minimum delay (current_min_rtt_) of the current
+ // burst of packets relative to the minimum delay during the session.
+ // Note: we only look at the first few(8) packets in each burst, since we
+ // only want to compare the lowest RTT of the burst relative to previous
+ // bursts.
+ rtt_sample_count_++;
+ if (rtt_sample_count_ <= kHybridStartMinSamples) {
+ if (current_min_rtt_.IsZero() || current_min_rtt_ > latest_rtt) {
+ current_min_rtt_ = latest_rtt;
+ }
+ }
+ // We only need to check this once per round.
+ if (rtt_sample_count_ == kHybridStartMinSamples) {
+ // Divide min_rtt by 8 to get a rtt increase threshold for exiting.
+ int64_t min_rtt_increase_threshold_us =
+ min_rtt.ToMicroseconds() >> kHybridStartDelayFactorExp;
+ // Ensure the rtt threshold is never less than 2ms or more than 16ms.
+ min_rtt_increase_threshold_us = std::min(min_rtt_increase_threshold_us,
+ kHybridStartDelayMaxThresholdUs);
+ QuicTime::Delta min_rtt_increase_threshold =
+ QuicTime::Delta::FromMicroseconds(std::max(
+ min_rtt_increase_threshold_us, kHybridStartDelayMinThresholdUs));
+
+ if (current_min_rtt_ > min_rtt + min_rtt_increase_threshold) {
+ hystart_found_ = DELAY;
+ }
+ }
+ // Exit from slow start if the cwnd is greater than 16 and
+ // increasing delay is found.
+ return congestion_window >= kHybridStartLowWindow &&
+ hystart_found_ != NOT_FOUND;
+}
+
+} // namespace quic
