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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 
     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 
     31 void HybridSlowStart::Restart() {
     32   found_ack_train_ = false;
     33   found_delay_  = false;
     34 }
     35 
     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 
     45 bool HybridSlowStart::EndOfRound(QuicPacketSequenceNumber ack) {
     46   return end_sequence_number_ <= ack;
     47 }
     48 
     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 
    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