rtt-estimator.cc

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//
// Copyright (c) 2006 Georgia Tech Research Corporation
//
// SPDX-License-Identifier: GPL-2.0-only
//
// Author: Rajib Bhattacharjea<raj.b@gatech.edu>
//
 
// Ported from:
// Georgia Tech Network Simulator - Round Trip Time Estimator Class
// George F. Riley.  Georgia Tech, Spring 2002
 
// Base class allows variations of round trip time estimators to be
// implemented
 
#include "rtt-estimator.h"
 
#include "ns3/double.h"
#include "ns3/log.h"
 
#include <cmath>
#include <iostream>
 
namespace ns3
{
 
NS_LOG_COMPONENT_DEFINE("RttEstimator");
 
NS_OBJECT_ENSURE_REGISTERED(RttEstimator);
 
/// Tolerance used to check reciprocal of two numbers.
static const double TOLERANCE = 1e-6;
 
TypeId
RttEstimator::GetTypeId()
{
    static TypeId tid = TypeId("ns3::RttEstimator")
                            .SetParent<Object>()
                            .SetGroupName("Internet")
                            .AddAttribute("InitialEstimation",
                                          "Initial RTT estimate",
                                          TimeValue(Seconds(1)),
                                          MakeTimeAccessor(&RttEstimator::m_initialEstimatedRtt),
                                          MakeTimeChecker());
    return tid;
}
 
Time
RttEstimator::GetEstimate() const
{
    return m_estimatedRtt;
}
 
Time
RttEstimator::GetVariation() const
{
    return m_estimatedVariation;
}
 
// Base class methods
 
RttEstimator::RttEstimator()
    : m_nSamples(0)
{
    NS_LOG_FUNCTION(this);
 
    // We need attributes initialized here, not later, so use the
    // ConstructSelf() technique documented in the manual
    ObjectBase::ConstructSelf(AttributeConstructionList());
    m_estimatedRtt = m_initialEstimatedRtt;
    m_estimatedVariation = Time(0);
    NS_LOG_DEBUG("Initialize m_estimatedRtt to " << m_estimatedRtt.GetSeconds() << " sec.");
}
 
RttEstimator::RttEstimator(const RttEstimator& c)
    : Object(c),
      m_initialEstimatedRtt(c.m_initialEstimatedRtt),
      m_estimatedRtt(c.m_estimatedRtt),
      m_estimatedVariation(c.m_estimatedVariation),
      m_nSamples(c.m_nSamples)
{
    NS_LOG_FUNCTION(this);
}
 
RttEstimator::~RttEstimator()
{
    NS_LOG_FUNCTION(this);
}
 
void
RttEstimator::Reset()
{
    NS_LOG_FUNCTION(this);
    // Reset to initial state
    m_estimatedRtt = m_initialEstimatedRtt;
    m_estimatedVariation = Time(0);
    m_nSamples = 0;
}
 
uint32_t
RttEstimator::GetNSamples() const
{
    return m_nSamples;
}
 
//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------
// Mean-Deviation Estimator
 
NS_OBJECT_ENSURE_REGISTERED(RttMeanDeviation);
 
TypeId
RttMeanDeviation::GetTypeId()
{
    static TypeId tid =
        TypeId("ns3::RttMeanDeviation")
            .SetParent<RttEstimator>()
            .SetGroupName("Internet")
            .AddConstructor<RttMeanDeviation>()
            .AddAttribute("Alpha",
                          "Gain used in estimating the RTT, must be 0 <= alpha <= 1",
                          DoubleValue(0.125),
                          MakeDoubleAccessor(&RttMeanDeviation::m_alpha),
                          MakeDoubleChecker<double>(0, 1))
            .AddAttribute("Beta",
                          "Gain used in estimating the RTT variation, must be 0 <= beta <= 1",
                          DoubleValue(0.25),
                          MakeDoubleAccessor(&RttMeanDeviation::m_beta),
                          MakeDoubleChecker<double>(0, 1));
    return tid;
}
 
RttMeanDeviation::RttMeanDeviation()
{
    NS_LOG_FUNCTION(this);
}
 
RttMeanDeviation::RttMeanDeviation(const RttMeanDeviation& c)
    : RttEstimator(c),
      m_alpha(c.m_alpha),
      m_beta(c.m_beta)
{
    NS_LOG_FUNCTION(this);
}
 
uint32_t
RttMeanDeviation::CheckForReciprocalPowerOfTwo(double val) const
{
    NS_LOG_FUNCTION(this << val);
    if (val < TOLERANCE)
    {
        return 0;
    }
    // supports 1/32, 1/16, 1/8, 1/4, 1/2
    if (std::abs(1 / val - 8) < TOLERANCE)
    {
        return 3;
    }
    if (std::abs(1 / val - 4) < TOLERANCE)
    {
        return 2;
    }
    if (std::abs(1 / val - 32) < TOLERANCE)
    {
        return 5;
    }
    if (std::abs(1 / val - 16) < TOLERANCE)
    {
        return 4;
    }
    if (std::abs(1 / val - 2) < TOLERANCE)
    {
        return 1;
    }
    return 0;
}
 
void
RttMeanDeviation::FloatingPointUpdate(Time m)
{
    NS_LOG_FUNCTION(this << m);
 
    // EWMA formulas are implemented as suggested in
    // Jacobson/Karels paper appendix A.2
 
    // SRTT <- (1 - alpha) * SRTT + alpha *  R'
    Time err(m - m_estimatedRtt);
    double gErr = err.ToDouble(Time::S) * m_alpha;
    m_estimatedRtt += Time::FromDouble(gErr, Time::S);
 
    // RTTVAR <- (1 - beta) * RTTVAR + beta * |SRTT - R'|
    Time difference = Abs(err) - m_estimatedVariation;
    m_estimatedVariation += difference * m_beta;
}
 
void
RttMeanDeviation::IntegerUpdate(Time m, uint32_t rttShift, uint32_t variationShift)
{
    NS_LOG_FUNCTION(this << m << rttShift << variationShift);
    // Jacobson/Karels paper appendix A.2
    int64_t meas = m.GetInteger();
    int64_t delta = meas - m_estimatedRtt.GetInteger();
    int64_t srtt = (m_estimatedRtt.GetInteger() << rttShift) + delta;
    m_estimatedRtt = Time::From(srtt >> rttShift);
    if (delta < 0)
    {
        delta = -delta;
    }
    delta -= m_estimatedVariation.GetInteger();
    int64_t rttvar = m_estimatedVariation.GetInteger() << variationShift;
    rttvar += delta;
    m_estimatedVariation = Time::From(rttvar >> variationShift);
}
 
void
RttMeanDeviation::Measurement(Time m)
{
    NS_LOG_FUNCTION(this << m);
    if (m_nSamples)
    {
        // If both alpha and beta are reciprocal powers of two, updating can
        // be done with integer arithmetic according to Jacobson/Karels paper.
        // If not, since class Time only supports integer multiplication,
        // must convert Time to floating point and back again
        uint32_t rttShift = CheckForReciprocalPowerOfTwo(m_alpha);
        uint32_t variationShift = CheckForReciprocalPowerOfTwo(m_beta);
        if (rttShift && variationShift)
        {
            IntegerUpdate(m, rttShift, variationShift);
        }
        else
        {
            FloatingPointUpdate(m);
        }
    }
    else
    {                                 // First sample
        m_estimatedRtt = m;           // Set estimate to current
        m_estimatedVariation = m / 2; // And variation to current / 2
        NS_LOG_DEBUG("(first sample) m_estimatedVariation += " << m);
    }
    m_nSamples++;
}
 
Ptr<RttEstimator>
RttMeanDeviation::Copy() const
{
    NS_LOG_FUNCTION(this);
    return CopyObject<RttMeanDeviation>(this);
}
 
void
RttMeanDeviation::Reset()
{
    NS_LOG_FUNCTION(this);
    RttEstimator::Reset();
}
 
} // namespace ns3