ns3学习（二）—

关于NewReno的理论部分学习可以参考拥塞控制算法（二）——Tahoe、Reno、NewReno、Vegas

一、ns3源码

在文件tcp-congestion-ops.cc文件中是TCP-NewReno的源码

/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
/** Copyright (c) 2015 Natale Patriciello <natale.patriciello@gmail.com>** This program is free software; you can redistribute it and/or modify* it under the terms of the GNU General Public License version 2 as* published by the Free Software Foundation;** This program is distributed in the hope that it will be useful,* but WITHOUT ANY WARRANTY; without even the implied warranty of* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the* GNU General Public License for more details.** You should have received a copy of the GNU General Public License* along with this program; if not, write to the Free Software* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA**/
#include "tcp-congestion-ops.h"
#include "tcp-socket-base.h"
#include "ns3/log.h"namespace ns3 {NS_LOG_COMPONENT_DEFINE ("TcpCongestionOps");NS_OBJECT_ENSURE_REGISTERED (TcpCongestionOps);TypeId
TcpCongestionOps::GetTypeId (void)
{static TypeId tid = TypeId ("ns3::TcpCongestionOps").SetParent<Object> ().SetGroupName ("Internet");return tid;
}TcpCongestionOps::TcpCongestionOps () : Object ()
{}TcpCongestionOps::TcpCongestionOps (const TcpCongestionOps &other) : Object (other)
{}TcpCongestionOps::~TcpCongestionOps ()
{}// RENONS_OBJECT_ENSURE_REGISTERED (TcpNewReno);TypeId
TcpNewReno::GetTypeId (void)
{static TypeId tid = TypeId ("ns3::TcpNewReno").SetParent<TcpCongestionOps> ().SetGroupName ("Internet").AddConstructor<TcpNewReno> ();return tid;
}TcpNewReno::TcpNewReno (void) : TcpCongestionOps ()
{NS_LOG_FUNCTION (this);
}TcpNewReno::TcpNewReno (const TcpNewReno& sock): TcpCongestionOps (sock)
{NS_LOG_FUNCTION (this);
}TcpNewReno::~TcpNewReno (void)
{}/*** \brief Tcp NewReno slow start algorithm** Defined in RFC 5681 as** > During slow start, a TCP increments cwnd by at most SMSS bytes for* > each ACK received that cumulatively acknowledges new data.  Slow* > start ends when cwnd exceeds ssthresh (or, optionally, when it* > reaches it, as noted above) or when congestion is observed.  While* > traditionally TCP implementations have increased cwnd by precisely* > SMSS bytes upon receipt of an ACK covering new data, we RECOMMEND* > that TCP implementations increase cwnd, per:* >* >    cwnd += min (N, SMSS)                      (2)* >* > where N is the number of previously unacknowledged bytes acknowledged* > in the incoming ACK.** The ns-3 implementation respect the RFC definition. Linux does something* different:* \verbatim
u32 tcp_slow_start(struct tcp_sock *tp, u32 acked){u32 cwnd = tp->snd_cwnd + acked;if (cwnd > tp->snd_ssthresh)cwnd = tp->snd_ssthresh + 1;acked -= cwnd - tp->snd_cwnd;tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);return acked;}\endverbatim** As stated, we want to avoid the case when a cumulative ACK increases cWnd more* than a segment size, but we keep count of how many segments we have ignored,* and return them.** \param tcb internal congestion state* \param segmentsAcked count of segments acked* \return the number of segments not considered for increasing the cWnd*/
uint32_t
TcpNewReno::SlowStart (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked)
{NS_LOG_FUNCTION (this << tcb << segmentsAcked);if (segmentsAcked >= 1){tcb->m_cWnd += tcb->m_segmentSize;NS_LOG_INFO ("In SlowStart, updated to cwnd " << tcb->m_cWnd << " ssthresh " << tcb->m_ssThresh);return segmentsAcked - 1;}return 0;
}/*** \brief NewReno congestion avoidance** During congestion avoidance, cwnd is incremented by roughly 1 full-sized* segment per round-trip time (RTT).** \param tcb internal congestion state* \param segmentsAcked count of segments acked*/
void
TcpNewReno::CongestionAvoidance (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked)
{NS_LOG_FUNCTION (this << tcb << segmentsAcked);if (segmentsAcked > 0){double adder = static_cast<double> (tcb->m_segmentSize * tcb->m_segmentSize) / tcb->m_cWnd.Get ();adder = std::max (1.0, adder);tcb->m_cWnd += static_cast<uint32_t> (adder);NS_LOG_INFO ("In CongAvoid, updated to cwnd " << tcb->m_cWnd <<" ssthresh " << tcb->m_ssThresh);}
}/*** \brief Try to increase the cWnd following the NewReno specification** \see SlowStart* \see CongestionAvoidance** \param tcb internal congestion state* \param segmentsAcked count of segments acked*/
void
TcpNewReno::IncreaseWindow (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked)
{NS_LOG_FUNCTION (this << tcb << segmentsAcked);if (tcb->m_cWnd < tcb->m_ssThresh){segmentsAcked = SlowStart (tcb, segmentsAcked);}if (tcb->m_cWnd >= tcb->m_ssThresh){CongestionAvoidance (tcb, segmentsAcked);}/* At this point, we could have segmentsAcked != 0. This because RFC says* that in slow start, we should increase cWnd by min (N, SMSS); if in* slow start we receive a cumulative ACK, it counts only for 1 SMSS of* increase, wasting the others.** // Incorrect assert, I am sorry* NS_ASSERT (segmentsAcked == 0);*/
}std::string
TcpNewReno::GetName () const
{return "TcpNewReno";
}uint32_t
TcpNewReno::GetSsThresh (Ptr<const TcpSocketState> state,uint32_t bytesInFlight)
{NS_LOG_FUNCTION (this << state << bytesInFlight);return std::max (2 * state->m_segmentSize, bytesInFlight / 2);
}Ptr<TcpCongestionOps>
TcpNewReno::Fork ()
{return CopyObject<TcpNewReno> (this);
}} // namespace ns3

二、学习内容

1、在命名空间中写，方便其他部分进行引用

namespace ns3 {

2、定义日志

NS_LOG_COMPONENT_DEFINE ("TcpCongestionOps");

3、定义一个TypeId类——TcpCongestionOps

关于这部分的c++语言结构看不懂可以参考：C++类构造函数学习

定义一个名为TcpCongestionOps的TypeId类，并定义其GetTyped函数。

在GetTyped函数内部，定义全局属性TypeId为"ns3::TcpCongestionOps"。set其父类为Object，所属分组为Internet（应该是上层文件夹名）。无其他属性Attribute。

NS_OBJECT_ENSURE_REGISTERED (TcpCongestionOps);TypeId
TcpCongestionOps::GetTypeId (void)
{static TypeId tid = TypeId ("ns3::TcpCongestionOps").SetParent<Object> ().SetGroupName ("Internet");return tid;
}

设置TcpCongestionOps类的构造方法TcpCongestionOps（void）。并赋值Object为空。

TcpCongestionOps::TcpCongestionOps () : Object ()
{}

设置TcpCongestionOps类的构造方法TcpCongestionOps（const TcpCongestionOps &other）。并传入名为other的输入的索引（对other值更改则调用函数时传入的变量也更改。参考链接：C++学习——&）。

TcpCongestionOps::TcpCongestionOps (const TcpCongestionOps &other) : Object (other)
{}

设置TcpCongestionOps类的析构函数TcpCongestionOps（）。相当于把该对象分解掉，释放内内存

TcpCongestionOps::~TcpCongestionOps ()
{}

4、定义一个TypeId类——TcpNewReno

类似第三部分定义TcpCongestionOps类的同样操作，定义TcpNewReno。

// RENONS_OBJECT_ENSURE_REGISTERED (TcpNewReno);TypeId
TcpNewReno::GetTypeId (void)
{static TypeId tid = TypeId ("ns3::TcpNewReno").SetParent<TcpCongestionOps> ().SetGroupName ("Internet").AddConstructor<TcpNewReno> ();return tid;
}TcpNewReno::TcpNewReno (void) : TcpCongestionOps ()
{NS_LOG_FUNCTION (this);
}TcpNewReno::TcpNewReno (const TcpNewReno& sock): TcpCongestionOps (sock)
{NS_LOG_FUNCTION (this);
}TcpNewReno::~TcpNewReno (void)
{}

5、TcpNewReno慢启动算法

uint32_t
TcpNewReno::SlowStart (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked)
{NS_LOG_FUNCTION (this << tcb << segmentsAcked);if (segmentsAcked >= 1){tcb->m_cWnd += tcb->m_segmentSize;NS_LOG_INFO ("In SlowStart, updated to cwnd " << tcb->m_cWnd << " ssthresh " << tcb->m_ssThresh);return segmentsAcked - 1;}return 0;
}

5.1、Ptr<TcpSocketState> tcb

声明一个TcpSocketState类型的变量的指针，该指针名叫tcb。

Ptr是一个声明指针的模板类，C++中为固定用法。Ptr模板类定义如下：

template <typename T> class Ptr

T为传入值，类型为typename，可见是一个类型名称。Ptr类为template <typename T>class，即T类型的模板类。

5.2、tcb->m_cWnd

tcb是TcpSocketState类型的变量的指针，TcpSocketState类型中有m_cWnd属性。tcb->m_cWnd是指通过tcb来索引该属性。

5.2、SlowStart算法

Log部分不再阐述，this << tcb << segmentsAcked的意思与cout相似。

算法的关键部分是：

  if (segmentsAcked >= 1){tcb->m_cWnd += tcb->m_segmentSize;return segmentsAcked - 1;}

当ack大于1时，将拥塞窗口的大小进行调整，增加值为段大小。

由此可见，就是对拥塞窗口增加一个段的大小。可见NewReno在慢启动阶段是线性增长。

6、TcpNewReno拥塞避免算法

void
TcpNewReno::CongestionAvoidance (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked)
{NS_LOG_FUNCTION (this << tcb << segmentsAcked);if (segmentsAcked > 0){double adder = static_cast<double> (tcb->m_segmentSize * tcb->m_segmentSize) / tcb->m_cWnd.Get ();adder = std::max (1.0, adder);tcb->m_cWnd += static_cast<uint32_t> (adder);NS_LOG_INFO ("In CongAvoid, updated to cwnd " << tcb->m_cWnd <<" ssthresh " << tcb->m_ssThresh);}
}

static_cast<double>的含义与double类似，但是运行时间比double更快。

忽略Log部分，算法的关键部分是：
adder是拥塞避免阶段加在cwnd上的增量。该增量是max(1.0，段大小的平方除以cwnd）。

7、TcpNewReno增窗

void
TcpNewReno::IncreaseWindow (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked)
{NS_LOG_FUNCTION (this << tcb << segmentsAcked);if (tcb->m_cWnd < tcb->m_ssThresh){segmentsAcked = SlowStart (tcb, segmentsAcked);}if (tcb->m_cWnd >= tcb->m_ssThresh){CongestionAvoidance (tcb, segmentsAcked);}

TcpNewReno使用IncreaseWindow把SlowStart和CongestionAvidance封装起来。
拥塞窗口在ssThresh之下时，为慢启动阶段，窗口大于ssTresh时，为拥塞避免阶段。

8、ssThresh设置

GetName和Fork函数不再赘述，下面来看ssTresh的设置。

uint32_t
TcpNewReno::GetSsThresh (Ptr<const TcpSocketState> state,uint32_t bytesInFlight)
{NS_LOG_FUNCTION (this << state << bytesInFlight);return std::max (2 * state->m_segmentSize, bytesInFlight / 2);
}

ssTresh为max (2, 拥塞时的cwnd/2)

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ns3学习（二）——tcp-NewReno