CSE4232/5232 Spring 2006 Exam 1, open book, open notes.  Name ______________

1. How many bytes represent one character in each of the following
encodings?  (2 pts each)

             ANSWER
             ------
ASCII        1
UTF-8        1 to 3
ISO8859_1    1
latin1       1
Unicode      2

2. Fill in the number which describes the purpose of each of the
following IP or TCP packet header fields (3 pts each).

                      ANSWER
                      ------
IP checksum              10    1. Total Transmission Length.
IP ID number              9    2. Identifies application protocol.
IP MF flag               19    3. Identifies transport layer protocol.
IP protocol field         3    4. Identifies data link protocol.
IP TTL                   18    5. Request to open connection.
                               6. Close connection with handshake.
TCP SYN flag              5    7. Close connection without handshake.
TCP PSH flag             12    8. Number of packets sent.
TCP RST flag              7    9. Identifier for packet reassembly.
TCP acknowledgement no.  13    10. Transmission error detection.
TCP window size          14    11. Packet contains urgent data.
                               12. Packet contains data.
                               13. Next sequence number I am expecting.
                               14. Number of bytes I can receive right now.
                               15. IP fragmentation not supported.
                               16. Number of packets I have received.
                               17. Size of payload.
                               18. Number of router hops allowed.
                               19. Not the last fragment.
                               20. Other (specify):

3. Write a java method that takes a hostname and port number of a server,
and a message to send to it, then returns the first line of the response.
Any exceptions should be passed to the caller.  For example (20 pts):

  // Prints "HTTP/1.1 200 OK" if all goes well
  System.out.println(getFromServer("www.google.com", 80, "GET /\r\n"));


  // ANSWER
  String getFromServer(String host, int port, String message) throws Exception {
    Socket s=new Socket(host, port);
    InputStream in=s.getInputStream();
    OutputStream out=s.getOutputStream();
    PrintWriter pw=new PrintWriter(new OutputStreamWriter(out));
    BufferedReader br=new BufferedReader(new InputStreamReader(in));
    pw.print(message);
    pw.flush();
    return br.readLine();
  }


4. Write a Counter server.  A Counter server listens on port 12345,
ignores any input, and outputs a count (as an ASCII string) of
the number of clients that have connected since it started, including
the current connection, then closes the connection.  Handle each client
in a separate thread.  For example, if the server was running locally
(40 pts, use back of page).

  System.out.println(getFromServer("localhost", 12345, ""));  // "1"
  System.out.println(getFromServer("localhost", 12345, ""));  // "2"


// ANSWER.  This solution uses just one class, although it might be more
// conventional to have a separate class for the client handler thread.
// In either case the count must be incremented before it is passed to 
// the thread so that two threads do not try to increment the
// count at the same time.  Alternatively, the count can be stored in a
// static variable accessible to all threads and the code to increment 
// it and read its value be synchronized.

import java.io.*;
import java.net.*;

public class CounterServer extends Thread {
  Socket socket;
  int count;

  // Run the server
  public static void main(String[] args) throws Exception {
    ServerSocket serverSocket=new ServerSocket(12345);
    for (int i=1; ; ++i) {
      Socket s=serverSocket.accept();
      new CounterServer(s, i);
    }
  }

  // Create a new thread to print i to client socket s
  CounterServer(Socket s, int i) throws Exception {
    socket=s;
    count=i;
    start();
  }    

  // Handle the client
  public void run() {
    try {
      PrintWriter pr=new PrintWriter(new OutputStreamWriter(socket.getOutputStream()));
      pr.println(count);
      pr.close();
    }
    catch (Exception x) {}
  }
}