UDP Socket API

Many network applications need a capability for two clients to ping each other to determine the round-trip time, loss rate, and bandwidth between them. For example, a skype client A pings another skype client B before deciding if A should use B as a relay. In this assignment, you will implement a ping server and a corresponding client.

The ping protocol allows a client machine to send a sequence of packets of data to a remote machine, and have the remote machine return the data back to the client (an action referred to as echoing).

You are given the starting code for the Ping server below. Your assignment is to write the Ping client and extend the Ping server.

Part I: Introduction

Basic Server Code

The following code is your starting code for a ping server. You should study this code carefully, as it will help you write your Ping client and extend the server code.

// PingServer.java
import java.io.*;
import java.net.*;
import java.util.*;

/*
* Server to process ping requests over UDP.
*/
public class PingServer
{
private static final double LOSS_RATE = 0.3;
private static final int AVERAGE_DELAY = 100; // milliseconds

public static void main(String[] args) throws Exception
{
// Get command line argument.
if (args.length != 1) {
System.out.println("Required arguments: port");
return;
}
int port = Integer.parseInt(args[0]);

// Create random number generator for use in simulating
// packet loss and network delay.
Random random = new Random();

// Create a datagram socket for receiving and sending UDP packets
// through the port specified on the command line.
DatagramSocket socket = new DatagramSocket(port);

// Processing loop.
while (true) {
// Create a datagram packet to hold incomming UDP packet.
DatagramPacket request = new DatagramPacket(new byte[1024], 1024);

// Block until the host receives a UDP packet.
socket.receive(request);

// Print the received data.
printData(request);

// Decide whether to reply, or simulate packet loss.
if (random.nextDouble() < LOSS_RATE) {
System.out.println(" Reply not sent.");
continue;
}

// Simulate prorogation delay.
Thread.sleep((int) (random.nextDouble() * 2 * AVERAGE_DELAY));

// Send reply.
InetAddress clientHost = request.getAddress();
int clientPort = request.getPort();
byte[] buf = request.getData();
DatagramPacket reply = new DatagramPacket(buf, buf.length, clientHost, clientPort);
socket.send(reply);

System.out.println(" Reply sent.");
}
}

/*
* Print ping data to the standard output stream.
*/
private static void printData(DatagramPacket request) throws Exception
{
// Obtain references to the packet's array of bytes.
byte[] buf = request.getData();

// Wrap the bytes in a byte array input stream,
// so that you can read the data as a stream of bytes.
ByteArrayInputStream bais = new ByteArrayInputStream(buf);

// Wrap the byte array output stream in an input stream reader,
// so you can read the data as a stream of **characters**: reader/writer handles characters
InputStreamReader isr = new InputStreamReader(bais);

// Wrap the input stream reader in a bufferred reader,
// so you can read the character data a line at a time.
// (A line is a sequence of chars terminated by any combination of r and n.)
BufferedReader br = new BufferedReader(isr);

// The message data is contained in a single line, so read this line.
String line = br.readLine();

// Print host address and data received from it.
System.out.println(
"Received from " +
request.getAddress().getHostAddress() +
": " +
new String(line) );
}
}

The server sits in an infinite loop listening for incoming UDP packets. When a packet comes in, the server simply sends the encapsulated data back to the client.

Packet Loss and Delay

UDP provides applications with an unreliable transport service, because messages may get lost in the network due to router queue overflows or other reasons. In contrast, TCP provides applications with a reliable transport service and takes care of any lost packets by retransmitting them until they are successfully received. Applications using UDP for communication must therefore implement any reliability they need separately in the application level (each application can implement a different policy, according to its specific needs).

Because packet loss is rare or even non-existent in typical campus networks, the server in this assignment injects artificial losses to simulate the effects of network packet losses. The server has a parameter LOSS_RATE that determines which percentage of packets should be lost.

The server also has another parameter AVERAGE_DELAY that is used to simulate the propagation delay. You should set AVERAGE_DELAY to a positive value when testing your client and server on the same machine, or when machines are close by on the network. You can set AVERAGE_DELAY to 0 to find out the true round trip times of your packets.

Compiling and Running Server

To compile the example server, do the following:

javac PingServer.java

To run the example server, do the following:

java PingServer port

where port is the port number the server listens on. Remember that you have to pick a port number greater than 1024, because only processes running with the root (administrator) privilege can bind to ports less than 1024.

Note: if you get a class not found error when running the above command, then you may need to tell Java to look in the current directory in order to resolve class references. In this case, the commands will be as follows:

java -classpath . PingServer port

Part II: Your Job

Message Format

The ping messages from a client to the server in this assignment are formatted in the following way.

PING sequence_number client_send_time passwd CRLF

where PING is the 4 ASCII characters, sequence_number is 2 bytes, and starts at 0 and increases by 1 for each successive ping message sent by the client; client_send_time is 8-byte local timestamp (in the format of Java currentTimeMill) encoding when the client sends the message; passwd is ASCII so that a server echoes only valid requests, and CRLF represents the carriage return and line feed characters that terminate the line.

The reply from a server to a ping message is formatted in the following way:

PINGECHO sequence_number client_send_time passwd CRLF

where PINGECHO is the 8 ASCII characters, sequence_number, client_send_time, and passwd are echos of client message.

It is recommended that you read Chapter 4 of Java Network Programming on i/o issues.

Your Client

You should write the client so that it sends 10 ping requests to the server, separated by one second (Hint: you can Use the Timer and TimerTask classes in java.util.) After sending each packet, the client waits up to one second to receive a reply. If one seconds goes by without a reply from the server, then the client assumes that its packet or the server's reply packet has been lost in the network.

You should write the client so that it starts with the following command:

java PingClient host port passwd

where host is the name of the computer the server is running on and port is the port number it is listening to. Note that you can run the client and server either on different machines or on the same machine.

The client should send 10 pings to the server. Because UDP is an unreliable protocol, some of the packets sent to the server may be lost, or some of the packets sent from server to client may be lost. For this reason, the client can not wait indefinitely for a reply to a ping message. You should have the client wait up to one second for a reply; if no reply is received, then the client should assume that the packet was lost during transmission across the network. You will need to research the API for DatagramSocket to find out how to set the timeout value on a datagram socket.

The output of your client should report the minimum, maximum, and average RTTs. It should also report the loss rate.

When developing your code, you should run the ping server on your machine, and test your client by sending packets to localhost (or, 127.0.0.1). After you have fully debugged your code, you should see how your application communicates across the network with a ping server run on a different machine.

Your Server

Please modify the server program to check that the password is valid. The valid password should be provided when the server starts:

java PingServer port passwd [-delay delay] [-loss loss]