net/quic/congestion_control/rtt_stats.h

   1 // Copyright 2014 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 // A convenience class to store rtt samples and calculate smoothed rtt.
   6
   7 #ifndef NET_QUIC_CONGESTION_CONTROL_RTT_STATS_H_
   8 #define NET_QUIC_CONGESTION_CONTROL_RTT_STATS_H_
   9
  10 #include <algorithm>
  11
  12 #include "base/basictypes.h"
  13 #include "net/quic/quic_protocol.h"
  14 #include "net/quic/quic_time.h"
  15
  16 namespace net {
  17
  18 namespace test {
  19 class RttStatsPeer;
  20 }  // namespace test
  21
  22 class NET_EXPORT_PRIVATE RttStats {
  23  public:
  24   RttStats();
  25
  26   // Updates the RTT from an incoming ack which is received |send_delta| after
  27   // the packet is sent and the peer reports the ack being delayed |ack_delay|.
  28   void UpdateRtt(QuicTime::Delta send_delta,
  29                  QuicTime::Delta ack_delay,
  30                  QuicTime now);
  31
  32   // Causes the smoothed_rtt to be increased to the latest_rtt if the latest_rtt
  33   // is larger. The mean deviation is increased to the most recent deviation if
  34   // it's larger.
  35   void ExpireSmoothedMetrics();
  36
  37   // Forces RttStats to sample a new recent min rtt within the next
  38   // |num_samples| UpdateRtt calls.
  39   void SampleNewRecentMinRtt(uint32 num_samples);
  40
  41   // Returns the EWMA smoothed RTT for the connection.
  42   // May return Zero if no valid updates have occurred.
  43   QuicTime::Delta smoothed_rtt() const {
  44     return smoothed_rtt_;
  45   }
  46
  47   int64 initial_rtt_us() const {
  48     return initial_rtt_us_;
  49   }
  50
  51   // Sets an initial RTT to be used for SmoothedRtt before any RTT updates.
  52   void set_initial_rtt_us(int64 initial_rtt_us) {
  53     if (initial_rtt_us <= 0) {
  54       LOG(DFATAL) << "Attempt to set initial rtt to <= 0.";
  55       return;
  56     }
  57     initial_rtt_us_ = initial_rtt_us;
  58   }
  59
  60   // The most recent rtt measurement.
  61   // May return Zero if no valid updates have occurred.
  62   QuicTime::Delta latest_rtt() const {
  63     return latest_rtt_;
  64   }
  65
  66   // Returns the min_rtt for the entire connection.
  67   // May return Zero if no valid updates have occurred.
  68   QuicTime::Delta min_rtt() const {
  69     return min_rtt_;
  70   }
  71
  72   // Returns the min_rtt since SampleNewRecentMinRtt has been called, or the
  73   // min_rtt for the entire connection if SampleNewMinRtt was never called.
  74   QuicTime::Delta recent_min_rtt() const {
  75     return recent_min_rtt_.rtt;
  76   }
  77
  78   QuicTime::Delta mean_deviation() const {
  79     return mean_deviation_;
  80   }
  81
  82   // Sets how old a recent min rtt sample can be.
  83   void set_recent_min_rtt_window(QuicTime::Delta recent_min_rtt_window) {
  84     recent_min_rtt_window_ = recent_min_rtt_window;
  85   }
  86
  87  private:
  88   friend class test::RttStatsPeer;
  89
  90   // Used to track a sampled RTT window.
  91   struct RttSample {
  92     RttSample() : rtt(QuicTime::Delta::Zero()), time(QuicTime::Zero()) { }
  93     RttSample(QuicTime::Delta rtt, QuicTime time) : rtt(rtt), time(time) { }
  94
  95     QuicTime::Delta rtt;
  96     QuicTime time;  // Time the rtt sample was recorded.
  97   };
  98
  99   // Implements the resampling algorithm and the windowed min rtt algorithm.
 100   void UpdateRecentMinRtt(QuicTime::Delta rtt_sample, QuicTime now);
 101
 102   QuicTime::Delta latest_rtt_;
 103   QuicTime::Delta min_rtt_;
 104   QuicTime::Delta smoothed_rtt_;
 105   // Mean RTT deviation during this session.
 106   // Approximation of standard deviation, the error is roughly 1.25 times
 107   // larger than the standard deviation, for a normally distributed signal.
 108   QuicTime::Delta mean_deviation_;
 109   int64 initial_rtt_us_;
 110
 111   RttSample new_min_rtt_;
 112   uint32 num_min_rtt_samples_remaining_;
 113
 114   // State variables for Kathleen Nichols MinRTT algorithm.
 115   QuicTime::Delta recent_min_rtt_window_;
 116   RttSample recent_min_rtt_;  // a in the windowed algorithm.
 117   RttSample half_window_rtt_;  // b in the sampled algorithm.
 118   RttSample quarter_window_rtt_;  // c in the sampled algorithm.
 119
 120   DISALLOW_COPY_AND_ASSIGN(RttStats);
 121 };
 122
 123 }  // namespace net
 124
 125 #endif  // NET_QUIC_CONGESTION_CONTROL_RTT_STATS_H_