A number of UDP echo client/server pairs communicating with each other

udp_cs_pairs.cc

/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
/*
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 * published by the Free Software Foundation;
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#include "ns3/core-module.h"
#include "ns3/network-module.h"
#include "ns3/internet-module.h"
#include "ns3/point-to-point-module.h"
#include "ns3/applications-module.h"
#include <ctime>

using namespace ns3;

NS_LOG_COMPONENT_DEFINE ("FirstScriptExample");

int
main (int argc, char *argv[])
{
  time_t start, nodes_created, end;
  time(&start);

  bool verbose = true;
  uint32_t numPairs = 1;

  CommandLine cmd;
  cmd.AddValue ("numPairs", "Number of pairs of nodes/devices", numPairs);
  cmd.AddValue ("verbose", "Tell echo applications to log if true", verbose);

  cmd.Parse (argc,argv);

  if (verbose)
    {
      LogComponentEnable ("UdpEchoClientApplication", LOG_LEVEL_INFO);
      LogComponentEnable ("UdpEchoServerApplication", LOG_LEVEL_INFO);
    }

  NodeContainer nodes;
  nodes.Create (2*numPairs);

  Ipv4AddressHelper address;
  address.SetBase ("10.0.0.0", "255.0.0.0");

  PointToPointHelper pointToPoint;
  pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
  pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

  NetDeviceContainer devices;

  InternetStackHelper stack;
  stack.Install (nodes);

  Ipv4InterfaceContainer interfaces;

  UdpEchoServerHelper echoServer (9);

  ApplicationContainer serverApps;
  for (uint32_t i = 0; i < numPairs; i++)
  {
    devices = pointToPoint.Install (nodes.Get(2*i), nodes.Get(2*i+1));
    interfaces = address.Assign (devices);

    serverApps = echoServer.Install (nodes.Get (2*i+1));
    serverApps.Start (Seconds (1.0));
    serverApps.Stop (Seconds (3.0));

    UdpEchoClientHelper echoClient (interfaces.GetAddress (1), 9);
    echoClient.SetAttribute ("MaxPackets", UintegerValue (10000));
    echoClient.SetAttribute ("Interval", TimeValue (Seconds (0.1)));
    echoClient.SetAttribute ("PacketSize", UintegerValue (1024));
 
    ApplicationContainer clientApps = echoClient.Install (nodes.Get (2*i));
    clientApps.Start (Seconds (2.0));
    clientApps.Stop (Seconds (3.0));
  }
  time(&nodes_created);

  double loadtime = difftime(nodes_created, start);

  //pointToPoint.EnablePcapAll("myfirst");

  Simulator::Run ();
  Simulator::Destroy ();
  time(&end);
  double endtime = difftime(end, start);
  printf("Total Time = %.2lf seconds\n", endtime);
  printf("Load Time = %.2lf seconds\n", loadtime);

  return 0;
}

Sorry Sir Iam new to ns3
UdpEchoServerHelper echoServer (9); what does 9 represents here ?

Is the port of the service