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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 namespace net {
10 
11 // Note(pwestin): the magic clamping numbers come from the original code in
12 // tcp_cubic.c.
13 // Number of delay samples for detecting the increase of delay.
14 const int kHybridStartMinSamples = 8;
15 const int kHybridStartDelayFactorExp = 4;  // 2^4 = 16
16 const int kHybridStartDelayMinThresholdUs = 2000;
17 const int kHybridStartDelayMaxThresholdUs = 16000;
18 
HybridSlowStart(const QuicClock * clock)19 HybridSlowStart::HybridSlowStart(const QuicClock* clock)
20     : clock_(clock),
21       started_(false),
22       found_ack_train_(false),
23       found_delay_(false),
24       round_start_(QuicTime::Zero()),
25       end_sequence_number_(0),
26       last_time_(QuicTime::Zero()),
27       sample_count_(0),
28       current_rtt_(QuicTime::Delta::Zero()) {
29 }
30 
Restart()31 void HybridSlowStart::Restart() {
32   found_ack_train_ = false;
33   found_delay_  = false;
34 }
35 
Reset(QuicPacketSequenceNumber end_sequence_number)36 void HybridSlowStart::Reset(QuicPacketSequenceNumber end_sequence_number) {
37   DVLOG(1) << "Reset hybrid slow start @" << end_sequence_number;
38   round_start_ = last_time_ = clock_->ApproximateNow();
39   end_sequence_number_ = end_sequence_number;
40   current_rtt_ = QuicTime::Delta::Zero();
41   sample_count_ = 0;
42   started_ = true;
43 }
44 
EndOfRound(QuicPacketSequenceNumber ack)45 bool HybridSlowStart::EndOfRound(QuicPacketSequenceNumber ack) {
46   return end_sequence_number_ <= ack;
47 }
48 
Update(QuicTime::Delta rtt,QuicTime::Delta delay_min)49 void HybridSlowStart::Update(QuicTime::Delta rtt, QuicTime::Delta delay_min) {
50   // The original code doesn't invoke this until we hit 16 packet per burst.
51   // Since the code handles lower than 16 grecefully and I removed that
52   // limit.
53   if (found_ack_train_ || found_delay_) {
54     return;
55   }
56   QuicTime current_time = clock_->ApproximateNow();
57 
58   // First detection parameter - ack-train detection.
59   // Since slow start burst out packets we can indirectly estimate the inter-
60   // arrival time by looking at the arrival time of the ACKs if the ACKs are
61   // spread out more then half the minimum RTT packets are beeing spread out
62   // more than the capacity.
63   // This first trigger will not come into play until we hit roughly 4.8 Mbit/s.
64   // TODO(pwestin): we need to make sure our pacing don't trigger this detector.
65   if (current_time.Subtract(last_time_).ToMicroseconds() <=
66       kHybridStartDelayMinThresholdUs) {
67     last_time_ = current_time;
68     if (current_time.Subtract(round_start_).ToMicroseconds() >=
69         (delay_min.ToMicroseconds() >> 1)) {
70       found_ack_train_ = true;
71     }
72   }
73   // Second detection parameter - delay increase detection.
74   // Compare the minimum delay (current_rtt_) of the current
75   // burst of packets relative to the minimum delay during the session.
76   // Note: we only look at the first few(8) packets in each burst, since we
77   // only want to compare the lowest RTT of the burst relative to previous
78   // bursts.
79   sample_count_++;
80   if (sample_count_ <= kHybridStartMinSamples) {
81     if (current_rtt_.IsZero() || current_rtt_ > rtt) {
82       current_rtt_ = rtt;
83     }
84   }
85   // We only need to check this once.
86   if (sample_count_ == kHybridStartMinSamples) {
87     int accepted_variance_us = delay_min.ToMicroseconds() >>
88         kHybridStartDelayFactorExp;
89     accepted_variance_us = std::min(accepted_variance_us,
90                                     kHybridStartDelayMaxThresholdUs);
91     QuicTime::Delta accepted_variance = QuicTime::Delta::FromMicroseconds(
92         std::max(accepted_variance_us, kHybridStartDelayMinThresholdUs));
93 
94     if (current_rtt_ > delay_min.Add(accepted_variance)) {
95       found_delay_ = true;
96     }
97   }
98 }
99 
Exit()100 bool HybridSlowStart::Exit() {
101   // If either one of the two conditions are met we exit from slow start
102   // immediately.
103   if (found_ack_train_ || found_delay_) {
104     return true;
105   }
106   return false;
107 }
108 
109 }  // namespace net
110