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| 1 // Copyright (c) 2012 The Chromium Authors. All rights reserved. | |
| 2 // Use of this source code is governed by a BSD-style license that can be | |
| 3 // found in the LICENSE file. | |
| 4 | |
| 5 #include "net/quic/congestion_control/hybrid_slow_start.h" | |
| 6 | |
| 7 #include <algorithm> | |
| 8 | |
| 9 using std::max; | |
| 10 using std::min; | |
| 11 | |
| 12 namespace net { | |
| 13 | |
| 14 // Note(pwestin): the magic clamping numbers come from the original code in | |
| 15 // tcp_cubic.c. | |
| 16 const int64 kHybridStartLowWindow = 16; | |
| 17 // Number of delay samples for detecting the increase of delay. | |
| 18 const uint32 kHybridStartMinSamples = 8; | |
| 19 // Exit slow start if the min rtt has increased by more than 1/8th. | |
| 20 const int kHybridStartDelayFactorExp = 3; // 2^3 = 8 | |
| 21 // The original paper specifies 2 and 8ms, but those have changed over time. | |
| 22 const int64 kHybridStartDelayMinThresholdUs = 4000; | |
| 23 const int64 kHybridStartDelayMaxThresholdUs = 16000; | |
| 24 | |
| 25 HybridSlowStart::HybridSlowStart(const QuicClock* clock) | |
| 26 : clock_(clock), | |
| 27 ack_train_detection_(true), | |
| 28 started_(false), | |
| 29 hystart_found_(NOT_FOUND), | |
| 30 last_sent_sequence_number_(0), | |
| 31 round_start_(QuicTime::Zero()), | |
| 32 end_sequence_number_(0), | |
| 33 last_close_ack_pair_time_(QuicTime::Zero()), | |
| 34 rtt_sample_count_(0), | |
| 35 current_min_rtt_(QuicTime::Delta::Zero()) { | |
| 36 } | |
| 37 | |
| 38 void HybridSlowStart::OnPacketAcked( | |
| 39 QuicPacketSequenceNumber acked_sequence_number, bool in_slow_start) { | |
| 40 // OnPacketAcked gets invoked after ShouldExitSlowStart, so it's best to end | |
| 41 // the round when the final packet of the burst is received and start it on | |
| 42 // the next incoming ack. | |
| 43 if (in_slow_start && IsEndOfRound(acked_sequence_number)) { | |
| 44 started_ = false; | |
| 45 } | |
| 46 } | |
| 47 | |
| 48 void HybridSlowStart::OnPacketSent(QuicPacketSequenceNumber sequence_number) { | |
| 49 last_sent_sequence_number_ = sequence_number; | |
| 50 } | |
| 51 | |
| 52 void HybridSlowStart::Restart() { | |
| 53 started_ = false; | |
| 54 hystart_found_ = NOT_FOUND; | |
| 55 } | |
| 56 | |
| 57 void HybridSlowStart::StartReceiveRound(QuicPacketSequenceNumber last_sent) { | |
| 58 DVLOG(1) << "Reset hybrid slow start @" << last_sent; | |
| 59 round_start_ = last_close_ack_pair_time_ = clock_->ApproximateNow(); | |
| 60 end_sequence_number_ = last_sent; | |
| 61 current_min_rtt_ = QuicTime::Delta::Zero(); | |
| 62 rtt_sample_count_ = 0; | |
| 63 started_ = true; | |
| 64 } | |
| 65 | |
| 66 bool HybridSlowStart::IsEndOfRound(QuicPacketSequenceNumber ack) const { | |
| 67 return end_sequence_number_ <= ack; | |
| 68 } | |
| 69 | |
| 70 bool HybridSlowStart::ShouldExitSlowStart(QuicTime::Delta latest_rtt, | |
| 71 QuicTime::Delta min_rtt, | |
| 72 int64 congestion_window) { | |
| 73 if (!started_) { | |
| 74 // Time to start the hybrid slow start. | |
| 75 StartReceiveRound(last_sent_sequence_number_); | |
| 76 } | |
| 77 if (hystart_found_ != NOT_FOUND) { | |
| 78 return true; | |
| 79 } | |
| 80 QuicTime current_time = clock_->ApproximateNow(); | |
| 81 | |
| 82 // First detection parameter - ack-train detection. | |
| 83 // Since slow start burst out packets we can indirectly estimate the inter- | |
| 84 // arrival time by looking at the arrival time of the ACKs if the ACKs are | |
| 85 // spread out more then half the minimum RTT packets are being spread out | |
| 86 // more than the capacity. | |
| 87 // This first trigger will not come into play until we hit roughly 9.6 Mbps | |
| 88 // with delayed acks (or 4.8Mbps without delayed acks) | |
| 89 if (ack_train_detection_ && | |
| 90 current_time.Subtract(last_close_ack_pair_time_).ToMicroseconds() <= | |
| 91 kHybridStartDelayMinThresholdUs) { | |
| 92 last_close_ack_pair_time_ = current_time; | |
| 93 if (current_time.Subtract(round_start_).ToMicroseconds() >= | |
| 94 min_rtt.ToMicroseconds() >> 1) { | |
| 95 hystart_found_ = ACK_TRAIN; | |
| 96 } | |
| 97 } else if (last_close_ack_pair_time_ == round_start_) { | |
| 98 // If the previous ack wasn't close, then move forward the round start time | |
| 99 // to the incoming ack. | |
| 100 last_close_ack_pair_time_ = round_start_ = current_time; | |
| 101 } | |
| 102 // Second detection parameter - delay increase detection. | |
| 103 // Compare the minimum delay (current_min_rtt_) of the current | |
| 104 // burst of packets relative to the minimum delay during the session. | |
| 105 // Note: we only look at the first few(8) packets in each burst, since we | |
| 106 // only want to compare the lowest RTT of the burst relative to previous | |
| 107 // bursts. | |
| 108 rtt_sample_count_++; | |
| 109 if (rtt_sample_count_ <= kHybridStartMinSamples) { | |
| 110 if (current_min_rtt_.IsZero() || current_min_rtt_ > latest_rtt) { | |
| 111 current_min_rtt_ = latest_rtt; | |
| 112 } | |
| 113 } | |
| 114 // We only need to check this once per round. | |
| 115 if (rtt_sample_count_ == kHybridStartMinSamples) { | |
| 116 // Divide min_rtt by 16 to get a rtt increase threshold for exiting. | |
| 117 int64 min_rtt_increase_threshold_us = min_rtt.ToMicroseconds() >> | |
| 118 kHybridStartDelayFactorExp; | |
| 119 // Ensure the rtt threshold is never less than 2ms or more than 16ms. | |
| 120 min_rtt_increase_threshold_us = min(min_rtt_increase_threshold_us, | |
| 121 kHybridStartDelayMaxThresholdUs); | |
| 122 QuicTime::Delta min_rtt_increase_threshold = | |
| 123 QuicTime::Delta::FromMicroseconds(max(min_rtt_increase_threshold_us, | |
| 124 kHybridStartDelayMinThresholdUs)); | |
| 125 | |
| 126 if (current_min_rtt_ > min_rtt.Add(min_rtt_increase_threshold)) { | |
| 127 hystart_found_= DELAY; | |
| 128 } | |
| 129 } | |
| 130 // Exit from slow start if the cwnd is greater than 16 and an ack train or | |
| 131 // increasing delay are found. | |
| 132 return congestion_window >= kHybridStartLowWindow && | |
| 133 hystart_found_ != NOT_FOUND; | |
| 134 } | |
| 135 | |
| 136 } // namespace net | |
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Issue 992733002:
Remove //net (except for Android test stuff) and sdch (Closed)
Base URL: git@github.com:domokit/mojo.git@master