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 // Copyright (c) 2016 The Chromium Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. #ifndef QUICHE_QUIC_CORE_CONGESTION_CONTROL_WINDOWED_FILTER_H_ #define QUICHE_QUIC_CORE_CONGESTION_CONTROL_WINDOWED_FILTER_H_ // Implements Kathleen Nichols' algorithm for tracking the minimum (or maximum) // estimate of a stream of samples over some fixed time interval. (E.g., // the minimum RTT over the past five minutes.) The algorithm keeps track of // the best, second best, and third best min (or max) estimates, maintaining an // invariant that the measurement time of the n'th best >= n-1'th best. // The algorithm works as follows. On a reset, all three estimates are set to // the same sample. The second best estimate is then recorded in the second // quarter of the window, and a third best estimate is recorded in the second // half of the window, bounding the worst case error when the true min is // monotonically increasing (or true max is monotonically decreasing) over the // window. // // A new best sample replaces all three estimates, since the new best is lower // (or higher) than everything else in the window and it is the most recent. // The window thus effectively gets reset on every new min. The same property // holds true for second best and third best estimates. Specifically, when a // sample arrives that is better than the second best but not better than the // best, it replaces the second and third best estimates but not the best // estimate. Similarly, a sample that is better than the third best estimate // but not the other estimates replaces only the third best estimate. // // Finally, when the best expires, it is replaced by the second best, which in // turn is replaced by the third best. The newest sample replaces the third // best. #include "quiche/quic/core/quic_time.h" namespace quic { // Compares two values and returns true if the first is less than or equal // to the second. template struct QUIC_EXPORT_PRIVATE MinFilter { bool operator()(const T& lhs, const T& rhs) const { return lhs <= rhs; } }; // Compares two values and returns true if the first is greater than or equal // to the second. template struct QUIC_EXPORT_PRIVATE MaxFilter { bool operator()(const T& lhs, const T& rhs) const { return lhs >= rhs; } }; // Use the following to construct a windowed filter object of type T. // For example, a min filter using QuicTime as the time type: // WindowedFilter, QuicTime, QuicTime::Delta> ObjectName; // A max filter using 64-bit integers as the time type: // WindowedFilter, uint64_t, int64_t> ObjectName; // Specifically, this template takes four arguments: // 1. T -- type of the measurement that is being filtered. // 2. Compare -- MinFilter or MaxFilter, depending on the type of filter // desired. // 3. TimeT -- the type used to represent timestamps. // 4. TimeDeltaT -- the type used to represent continuous time intervals between // two timestamps. Has to be the type of (a - b) if both |a| and |b| are // of type TimeT. template class QUIC_EXPORT_PRIVATE WindowedFilter { public: // |window_length| is the period after which a best estimate expires. // |zero_value| is used as the uninitialized value for objects of T. // Importantly, |zero_value| should be an invalid value for a true sample. WindowedFilter(TimeDeltaT window_length, T zero_value, TimeT zero_time) : window_length_(window_length), zero_value_(zero_value), zero_time_(zero_time), estimates_{Sample(zero_value_, zero_time), Sample(zero_value_, zero_time), Sample(zero_value_, zero_time)} {} // Changes the window length. Does not update any current samples. void SetWindowLength(TimeDeltaT window_length) { window_length_ = window_length; } // Updates best estimates with |sample|, and expires and updates best // estimates as necessary. void Update(T new_sample, TimeT new_time) { // Reset all estimates if they have not yet been initialized, if new sample // is a new best, or if the newest recorded estimate is too old. if (estimates_[0].sample == zero_value_ || Compare()(new_sample, estimates_[0].sample) || new_time - estimates_[2].time > window_length_) { Reset(new_sample, new_time); return; } if (Compare()(new_sample, estimates_[1].sample)) { estimates_[1] = Sample(new_sample, new_time); estimates_[2] = estimates_[1]; } else if (Compare()(new_sample, estimates_[2].sample)) { estimates_[2] = Sample(new_sample, new_time); } // Expire and update estimates as necessary. if (new_time - estimates_[0].time > window_length_) { // The best estimate hasn't been updated for an entire window, so promote // second and third best estimates. estimates_[0] = estimates_[1]; estimates_[1] = estimates_[2]; estimates_[2] = Sample(new_sample, new_time); // Need to iterate one more time. Check if the new best estimate is // outside the window as well, since it may also have been recorded a // long time ago. Don't need to iterate once more since we cover that // case at the beginning of the method. if (new_time - estimates_[0].time > window_length_) { estimates_[0] = estimates_[1]; estimates_[1] = estimates_[2]; } return; } if (estimates_[1].sample == estimates_[0].sample && new_time - estimates_[1].time > window_length_ >> 2) { // A quarter of the window has passed without a better sample, so the // second-best estimate is taken from the second quarter of the window. estimates_[2] = estimates_[1] = Sample(new_sample, new_time); return; } if (estimates_[2].sample == estimates_[1].sample && new_time - estimates_[2].time > window_length_ >> 1) { // We've passed a half of the window without a better estimate, so take // a third-best estimate from the second half of the window. estimates_[2] = Sample(new_sample, new_time); } } // Resets all estimates to new sample. void Reset(T new_sample, TimeT new_time) { estimates_[0] = estimates_[1] = estimates_[2] = Sample(new_sample, new_time); } void Clear() { Reset(zero_value_, zero_time_); } T GetBest() const { return estimates_[0].sample; } T GetSecondBest() const { return estimates_[1].sample; } T GetThirdBest() const { return estimates_[2].sample; } private: struct QUIC_EXPORT_PRIVATE Sample { T sample; TimeT time; Sample(T init_sample, TimeT init_time) : sample(init_sample), time(init_time) {} }; TimeDeltaT window_length_; // Time length of window. T zero_value_; // Uninitialized value of T. TimeT zero_time_; // Uninitialized value of TimeT. Sample estimates_[3]; // Best estimate is element 0. }; } // namespace quic #endif // QUICHE_QUIC_CORE_CONGESTION_CONTROL_WINDOWED_FILTER_H_