官方给出的定义如下：

brief Congestion control abstract class

The design is inspired on what Linux v4.0 does (but it has been in place since years). The congestion control is split from the main socket code, and it is a pluggable component. An interface has been defined; variables are maintained in the TcpSocketState class, while subclasses of TcpCongestionOps operate over an instance of that class.

Only three methods has been utilized right now; however, Linux has many others, which can be added later in ns-3.

即，拥塞控制从主套接字代码中分离出来，它是成为了一个可插入的组件。其变量在TcpSocketState类中维护，而TcpCongestionOps的子类需要基于该类的实例运行。

在ns3的源代码中，实现了一个拥塞控制相关的类，名字叫做TcpCongestionOps，这个类实现于ns3/src/internet/model目录下的tcp-congestion-ops.cc文件中，继承于Object类。

在该类中有很多拥塞控制相关的函数，下面挑选几个进行介绍。

GetName

这个函数能够简单地得到一个字符串，表明了当前使用的拥塞控制算法的名称：

/**
* \brief Get the name of the congestion control algorithm
*
* \return A string identifying the name
*/
virtual std::string GetName () const = 0;

GetSsThresh

在一个丢包事件发生后，得到新的阈值。

这个函数保证了拥塞控制状态会在调用这个方法之前就得到了改变。

实现者应当返回慢启动阈值（并不是直接改变它），后面的函数会将返回的慢启动阈值调整为新的慢启动阈值。

  /**
   * \brief Get the slow start threshold after a loss event
   *
   * Is guaranteed that the congestion control state (TcpAckState_t) is
   * changed BEFORE the invocation of this method.
   * The implementator should return the slow start threshold (and not change
   * it directly) because, in the future, the TCP implementation may require to
   * instantly recover from a loss event (e.g. when there is a network with an high
   * reordering factor).
   *
   * \param tcb internal congestion state
   * \param bytesInFlight total bytes in flight
   * \return Slow start threshold
   */
  virtual uint32_t GetSsThresh (Ptr<const TcpSocketState> tcb,
                                uint32_t bytesInFlight) = 0;

IncreaseWindow

简单的拥塞避免算法。

这个函数允许直接拥塞窗口和阈值的大小。

  /**
   * \brief Congestion avoidance algorithm implementation
   *
   * Mimic the function cong_avoid in Linux. New segments have been ACKed,
   * and the congestion control duty is to set
   *
   * The function is allowed to change directly cWnd and/or ssThresh.
   *
   * \param tcb internal congestion state
   * \param segmentsAcked count of segments acked
   */
  virtual void IncreaseWindow (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked) = 0;

PktsAcked

简单来说，这个函数是关于对于接收到的ACK的一些计时的信息。

当接收到一个ACK时，并且包含计时信息。这个函数是可选的，即拥塞控制可以选择不去实现它，默认的实现什么都不去做。官方给出的解释如下：

  /**
   * \brief Timing information on received ACK
   *
   * The function is called every time an ACK is received (only one time
   * also for cumulative ACKs) and contains timing information. It is
   * optional (congestion controls can not implement it) and the default
   * implementation does nothing.
   *
   * \param tcb internal congestion state
   * \param segmentsAcked count of segments acked
   * \param rtt last rtt
   */
  virtual void PktsAcked (Ptr<TcpSocketState> tcb, uint32_t segmentsAcked,
                          const Time& rtt)

CongestionStateSet

简单来说，这个函数用来切换状态。

官方给出的解释如下：

  /**
   * \brief Trigger events/calculations specific to a congestion state
   *
   * This function mimics the function set_state in Linux.
   * The function is called before changing congestion state.
   *
   * \param tcb internal congestion state
   * \param newState new congestion state to which the TCP is going to switch
   */
  virtual void CongestionStateSet (Ptr<TcpSocketState> tcb,
                                   const TcpSocketState::TcpCongState_t newState)

CwndEvent

简单来说，当拥塞窗口事件发生时，触发计算、事件。

这个函数有两个参数，第一个为当前内部的拥塞状态，第二个为触发这个函数的事件。

官方给出的解释如下：

  /**
   * \brief Trigger events/calculations on occurrence congestion window event
   *
   * This function mimics the function cwnd_event in Linux.
   * The function is called in case of congestion window events.
   *
   * \param tcb internal congestion state
   * \param event the event which triggered this function
   */
  virtual void CwndEvent (Ptr<TcpSocketState> tcb,
                          const TcpSocketState::TcpCAEvent_t event)
  {
    NS_UNUSED (tcb);
    NS_UNUSED (event);
  }