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      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   // Returns true if any RTT measurements have been made.
     27   bool HasUpdates() const;
     28 
     29   // Updates the RTT from an incoming ack which is received |send_delta| after
     30   // the packet is sent and the peer reports the ack being delayed |ack_delay|.
     31   void UpdateRtt(QuicTime::Delta send_delta,
     32                  QuicTime::Delta ack_delay,
     33                  QuicTime now);
     34 
     35   // Forces RttStats to sample a new recent min rtt within the next
     36   // |num_samples| UpdateRtt calls.
     37   void SampleNewRecentMinRtt(uint32 num_samples);
     38 
     39   QuicTime::Delta SmoothedRtt() const;
     40 
     41   int64 initial_rtt_us() const {
     42     return initial_rtt_us_;
     43   }
     44 
     45   // Sets an initial RTT to be used for SmoothedRtt before any RTT updates.
     46   void set_initial_rtt_us(int64 initial_rtt_us) {
     47     initial_rtt_us_ = initial_rtt_us;
     48   }
     49 
     50   QuicTime::Delta latest_rtt() const {
     51     return latest_rtt_;
     52   }
     53 
     54   // Returns the min_rtt for the entire connection.
     55   QuicTime::Delta min_rtt() const {
     56     return min_rtt_;
     57   }
     58 
     59   // Returns the min_rtt since SampleNewRecentMinRtt has been called, or the
     60   // min_rtt for the entire connection if SampleNewMinRtt was never called.
     61   QuicTime::Delta recent_min_rtt() const {
     62     return recent_min_rtt_.rtt;
     63   }
     64 
     65   QuicTime::Delta mean_deviation() const {
     66     return mean_deviation_;
     67   }
     68 
     69   // Sets how old a recent min rtt sample can be.
     70   void set_recent_min_rtt_window(QuicTime::Delta recent_min_rtt_window) {
     71     recent_min_rtt_window_ = recent_min_rtt_window;
     72   }
     73 
     74  private:
     75   friend class test::RttStatsPeer;
     76 
     77   // Used to track a sampled RTT window.
     78   struct RttSample {
     79     RttSample() : rtt(QuicTime::Delta::Zero()), time(QuicTime::Zero()) { }
     80     RttSample(QuicTime::Delta rtt, QuicTime time) : rtt(rtt), time(time) { }
     81 
     82     QuicTime::Delta rtt;
     83     QuicTime time;  // Time the rtt sample was recorded.
     84   };
     85 
     86   // Implements the resampling algorithm and the windowed min rtt algorithm.
     87   void UpdateRecentMinRtt(QuicTime::Delta rtt_sample, QuicTime now);
     88 
     89   QuicTime::Delta latest_rtt_;
     90   QuicTime::Delta min_rtt_;
     91   QuicTime::Delta smoothed_rtt_;
     92   // Mean RTT deviation during this session.
     93   // Approximation of standard deviation, the error is roughly 1.25 times
     94   // larger than the standard deviation, for a normally distributed signal.
     95   QuicTime::Delta mean_deviation_;
     96   int64 initial_rtt_us_;
     97 
     98   RttSample new_min_rtt_;
     99   uint32 num_min_rtt_samples_remaining_;
    100 
    101   // State variables for Kathleen Nichols MinRTT algorithm.
    102   QuicTime::Delta recent_min_rtt_window_;
    103   RttSample recent_min_rtt_;  // a in the windowed algorithm.
    104   RttSample half_window_rtt_;  // b in the sampled algorithm.
    105   RttSample quarter_window_rtt_;  // c in the sampled algorithm.
    106 
    107   DISALLOW_COPY_AND_ASSIGN(RttStats);
    108 };
    109 
    110 }  // namespace net
    111 
    112 #endif  // NET_QUIC_CONGESTION_CONTROL_RTT_STATS_H_
    113