tcp-bic.cc

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/*
 * Copyright (c) 2014 Natale Patriciello <natale.patriciello@gmail.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 */
#include "tcp-bic.h"
 
#include "ns3/log.h"
#include "ns3/simulator.h"
 
namespace ns3
{
 
NS_LOG_COMPONENT_DEFINE("TcpBic");
NS_OBJECT_ENSURE_REGISTERED(TcpBic);
 
TypeId
TcpBic::GetTypeId()
{
    static TypeId tid =
        TypeId("ns3::TcpBic")
            .SetParent<TcpCongestionOps>()
            .AddConstructor<TcpBic>()
            .SetGroupName("Internet")
            .AddAttribute("FastConvergence",
                          "Turn on/off fast convergence.",
                          BooleanValue(true),
                          MakeBooleanAccessor(&TcpBic::m_fastConvergence),
                          MakeBooleanChecker())
            .AddAttribute("Beta",
                          "Beta for multiplicative decrease",
                          DoubleValue(0.8),
                          MakeDoubleAccessor(&TcpBic::m_beta),
                          MakeDoubleChecker<double>(0.0))
            .AddAttribute("MaxIncr",
                          "Limit on increment allowed during binary search",
                          UintegerValue(16),
                          MakeUintegerAccessor(&TcpBic::m_maxIncr),
                          MakeUintegerChecker<uint32_t>(1))
            .AddAttribute("LowWnd",
                          "Threshold window size (in segments) for engaging BIC response",
                          UintegerValue(14),
                          MakeUintegerAccessor(&TcpBic::m_lowWnd),
                          MakeUintegerChecker<uint32_t>())
            .AddAttribute("SmoothPart",
                          "Number of RTT needed to approach cWnd_max from "
                          "cWnd_max-BinarySearchCoefficient. It can be viewed as the gradient "
                          "of the slow start AIM phase: less this value is, "
                          "more steep the increment will be.",
                          UintegerValue(5),
                          MakeUintegerAccessor(&TcpBic::m_smoothPart),
                          MakeUintegerChecker<uint32_t>(1))
            .AddAttribute("BinarySearchCoefficient",
                          "Inverse of the coefficient for the "
                          "binary search. Default 4, as in Linux",
                          UintegerValue(4),
                          MakeUintegerAccessor(&TcpBic::m_b),
                          MakeUintegerChecker<uint8_t>(2));
    return tid;
}
 
TcpBic::TcpBic()
    : TcpCongestionOps(),
      m_cWndCnt(0),
      m_lastMaxCwnd(0),
      m_lastCwnd(0),
      m_epochStart(Time::Min())
{
    NS_LOG_FUNCTION(this);
}
 
TcpBic::TcpBic(const TcpBic& sock)
    : TcpCongestionOps(sock),
      m_fastConvergence(sock.m_fastConvergence),
      m_beta(sock.m_beta),
      m_maxIncr(sock.m_maxIncr),
      m_lowWnd(sock.m_lowWnd),
      m_smoothPart(sock.m_smoothPart),
      m_cWndCnt(sock.m_cWndCnt),
      m_lastMaxCwnd(sock.m_lastMaxCwnd),
      m_lastCwnd(sock.m_lastCwnd),
      m_epochStart(sock.m_epochStart),
      m_b(sock.m_b)
{
    NS_LOG_FUNCTION(this);
}
 
void
TcpBic::IncreaseWindow(Ptr<TcpSocketState> tcb, uint32_t segmentsAcked)
{
    NS_LOG_FUNCTION(this << tcb << segmentsAcked);
 
    if (tcb->m_cWnd < tcb->m_ssThresh)
    {
        tcb->m_cWnd += tcb->m_segmentSize;
        segmentsAcked -= 1;
 
        NS_LOG_INFO("In SlowStart, updated to cwnd " << tcb->m_cWnd << " ssthresh "
                                                     << tcb->m_ssThresh);
    }
 
    if (tcb->m_cWnd >= tcb->m_ssThresh && segmentsAcked > 0)
    {
        m_cWndCnt += segmentsAcked;
        uint32_t cnt = Update(tcb);
 
        /* According to the BIC paper and RFC 6356 even once the new cwnd is
         * calculated you must compare this to the number of ACKs received since
         * the last cwnd update. If not enough ACKs have been received then cwnd
         * cannot be updated.
         */
        if (m_cWndCnt > cnt)
        {
            tcb->m_cWnd += tcb->m_segmentSize;
            m_cWndCnt = 0;
            NS_LOG_INFO("In CongAvoid, updated to cwnd " << tcb->m_cWnd);
        }
        else
        {
            NS_LOG_INFO("Not enough segments have been ACKed to increment cwnd."
                        "Until now "
                        << m_cWndCnt);
        }
    }
}
 
uint32_t
TcpBic::Update(Ptr<TcpSocketState> tcb)
{
    NS_LOG_FUNCTION(this << tcb);
 
    uint32_t segCwnd = tcb->GetCwndInSegments();
    uint32_t cnt;
 
    m_lastCwnd = segCwnd;
 
    if (m_epochStart == Time::Min())
    {
        m_epochStart = Simulator::Now(); /* record the beginning of an epoch */
    }
 
    if (segCwnd < m_lowWnd)
    {
        NS_LOG_INFO("Under lowWnd, compatibility mode. Behaving as NewReno");
        cnt = segCwnd;
        return cnt;
    }
 
    if (segCwnd < m_lastMaxCwnd)
    {
        double dist = (m_lastMaxCwnd - segCwnd) / m_b;
 
        NS_LOG_INFO("cWnd = " << segCwnd << " under lastMax, " << m_lastMaxCwnd
                              << " and dist=" << dist);
        if (dist > m_maxIncr)
        {
            /* Linear increase */
            cnt = segCwnd / m_maxIncr;
            NS_LOG_INFO("Linear increase (maxIncr=" << m_maxIncr << "), cnt=" << cnt);
        }
        else if (dist <= 1)
        {
            /* smoothed binary search increase: when our window is really
             * close to the last maximum, we parameterize in m_smoothPart the number
             * of RTT needed to reach that window.
             */
            cnt = (segCwnd * m_smoothPart) / m_b;
 
            NS_LOG_INFO("Binary search increase (smoothPart=" << m_smoothPart << "), cnt=" << cnt);
        }
        else
        {
            /* binary search increase */
            cnt = static_cast<uint32_t>(segCwnd / dist);
 
            NS_LOG_INFO("Binary search increase, cnt=" << cnt);
        }
    }
    else
    {
        NS_LOG_INFO("cWnd = " << segCwnd << " above last max, " << m_lastMaxCwnd);
        if (segCwnd < m_lastMaxCwnd + m_b)
        {
            /* slow start AMD linear increase */
            cnt = (segCwnd * m_smoothPart) / m_b;
            NS_LOG_INFO("Slow start AMD, cnt=" << cnt);
        }
        else if (segCwnd < m_lastMaxCwnd + m_maxIncr * (m_b - 1))
        {
            /* slow start */
            cnt = (segCwnd * (m_b - 1)) / (segCwnd - m_lastMaxCwnd);
 
            NS_LOG_INFO("Slow start, cnt=" << cnt);
        }
        else
        {
            /* linear increase */
            cnt = segCwnd / m_maxIncr;
 
            NS_LOG_INFO("Linear, cnt=" << cnt);
        }
    }
 
    /* if in slow start or link utilization is very low. Code taken from Linux
     * kernel, not sure of the source they take it. Usually, it is not reached,
     * since if m_lastMaxCwnd is 0, we are (hopefully) in slow start.
     */
    if (m_lastMaxCwnd == 0)
    {
        if (cnt > 20) /* increase cwnd 5% per RTT */
        {
            cnt = 20;
        }
    }
 
    if (cnt == 0)
    {
        cnt = 1;
    }
 
    return cnt;
}
 
std::string
TcpBic::GetName() const
{
    return "TcpBic";
}
 
uint32_t
TcpBic::GetSsThresh(Ptr<const TcpSocketState> tcb, uint32_t bytesInFlight)
{
    NS_LOG_FUNCTION(this);
 
    uint32_t segCwnd = tcb->GetCwndInSegments();
    uint32_t ssThresh = 0;
 
    m_epochStart = Time::Min();
 
    /* Wmax and fast convergence */
    if (segCwnd < m_lastMaxCwnd && m_fastConvergence)
    {
        NS_LOG_INFO("Fast Convergence. Last max cwnd: " << m_lastMaxCwnd << " updated to "
                                                        << static_cast<uint32_t>(m_beta * segCwnd));
        m_lastMaxCwnd = static_cast<uint32_t>(m_beta * segCwnd);
    }
    else
    {
        NS_LOG_INFO("Last max cwnd: " << m_lastMaxCwnd << " updated to " << segCwnd);
        m_lastMaxCwnd = segCwnd;
    }
 
    if (segCwnd < m_lowWnd)
    {
        ssThresh = std::max(2 * tcb->m_segmentSize, bytesInFlight / 2);
        NS_LOG_INFO("Less than lowWindow, ssTh= " << ssThresh);
    }
    else
    {
        ssThresh = static_cast<uint32_t>(std::max(segCwnd * m_beta, 2.0) * tcb->m_segmentSize);
        NS_LOG_INFO("More than lowWindow, ssTh= " << ssThresh);
    }
 
    return ssThresh;
}
 
Ptr<TcpCongestionOps>
TcpBic::Fork()
{
    return CopyObject<TcpBic>(this);
}
 
} // namespace ns3