15.4 Adding a sample script

At this point, one may want to try to take the basic scaffolding defined above and add it into the system. Performing this step now allows one to use a simpler model when plumbing into the system and may also reveal whether any design or API modifications need to be made. Once this is done, we will return to building out the functionality of the ErrorModels themselves.

15.4.1 Add basic support in the class

point-to-point-net-device.h
  class ErrorModel;
  
  /**
   * Error model for receive packet events
   */
  Ptr<ErrorModel> m_receiveErrorModel;
  

15.4.2 Add Accessor

  void
PointToPointNetDevice::SetReceiveErrorModel (Ptr<ErrorModel> em)
{
  NS_LOG_FUNCTION (this << em);
  m_receiveErrorModel = em;
} 

   .AddAttribute ("ReceiveErrorModel",
                   "The receiver error model used to simulate packet loss",
                   PointerValue (),
                   MakePointerAccessor (&PointToPointNetDevice::m_receiveErrorModel),
                   MakePointerChecker<ErrorModel> ())

15.4.3 Plumb into the system

void PointToPointNetDevice::Receive (Ptr<Packet> packet)
{
  NS_LOG_FUNCTION (this << packet);
  uint16_t protocol = 0;
  
  if (m_receiveErrorModel && m_receiveErrorModel->IsCorrupt (packet) )
    {
//  
// If we have an error model and it indicates that it is time to lose a
// corrupted packet, don't forward this packet up, let it go. 
// 
      m_dropTrace (packet);
    } 
  else
    {
//  
// Hit the receive trace hook, strip off the point-to-point protocol header
// and forward this packet up the protocol stack.
//     
      m_rxTrace (packet);
      ProcessHeader(packet, protocol);
      m_rxCallback (this, packet, protocol, GetRemote ());
      if (!m_promiscCallback.IsNull ())
        {           m_promiscCallback (this, packet, protocol, GetRemote (), GetAddress (), NetDevice::PACKET_HOST);
        }
    }
}

15.4.4 Create null functional script

simple-error-model.cc

  // Error model
  // We want to add an error model to node 3's NetDevice
  // We can obtain a handle to the NetDevice via the channel and node
  // pointers
  Ptr<PointToPointNetDevice> nd3 = PointToPointTopology::GetNetDevice
    (n3, channel2);
  Ptr<ErrorModel> em = Create<ErrorModel> ();
  nd3->SetReceiveErrorModel (em);


bool
ErrorModel::DoCorrupt (Packet& p)
{
  NS_LOG_FUNCTION;
  NS_LOG_UNCOND("Corrupt!");
  return false;
}

At this point, we can run the program with our trivial ErrorModel plumbed into the receive path of the PointToPointNetDevice. It prints out the string "Corrupt!" for each packet received at node n3. Next, we return to the error model to add in a subclass that performs more interesting error modeling.

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