c++ - How can I choose the port to which I send the response

Question

I am currently trying to communicate with a PLC by using Modbus/TCP but even thought I can read the Modbus frame send by the PLC I must give the Modbus response to port 502 and the stream I use send to the port used by the PLC to send the frame. I tried using two stream to receive and send but when i close the first one the PLC get that as a timed out communication and then refuse the second connection. If it can help you here is the code I use but for now it barely does anything else than allowing me to test the connection.

#define _WIN32_WINNT 0x0501

#include <iostream>
#include <boost/asio.hpp>
#include <boost/system/config.hpp>
#include <string>

using namespace std;
using boost::asio::ip::tcp;


int main()
{
    boost::asio::io_service io_service;

    tcp::endpoint endpoint(tcp::v4(), 502);
    tcp::acceptor acceptor(io_service, endpoint);

    while (1)
    {
        int i =0;
        string buff;
        char buf[30];
        tcp::iostream stream;
        tcp::iostream s("192.168.10.150", "502");
      acceptor.accept(*stream.rdbuf());
      getline(stream, buff);
          cout<<buff<<endl;
        s <<buff;
    }
}

If you have any suggestion.