The differences show up most in error handling. Packets can, like postcards, go astray. When was the last time the postman rang your bell to tell you that the post office had lost one of several postcards it was supposed to deliver to you? It doesn't happen, right? Because they don't keep track of them. On the other hand, when you're talking on the phone and there's a noise burst -- like somebody yelling in the room, or even a bad connection -- you can ask the person at the other end to repeat what they just said.

With a stream-based connection like a TCP socket, the network transport layer handles errors for you: it asks the other end to retransmit. With a datagram transport such as UDP, you have to handle retransmission yourself. Kind of like numbering the postcards you send, so that you can go back and resend any that don't arrive -- a good excuse to return to your vacation spot, perhaps.

Ian's Basic Steps: UDP Client UDP is a bit more involved, so I'll list the basic steps for generating a UDP client: Create a DatagramSocket with no arguments (the form that takes two arguments is used on the server). Optionally connect( ) the socket to an InetAddress (see Section 15.3) and port number. Create one or more DatagramPacket objects; these are wrappers around a byte array that contains data you want to send and is filled in with data you receive. If you did not connect( ) the socket, provide the InetAddress and port when constructing the DatagramPacket. Set the packet's length, and use sock.send(packet) to send data to the server. Use sock.receive( ) to retrieve data.

public class DaytimeUDP {
    /** The UDP port number */
    public final static int DAYTIME_PORT = 13;

    /** A buffer plenty big enough for the date string */
    protected final static int PACKET_SIZE = 100;

    // main program
    public static void main(String[] argv) throws IOException {
        if (argv.length <1) {
            System.err.println("usage: java DayTime host");
            System.exit(1);
        }
        String host = argv[0];
        InetAddress servAddr = InetAddress.getByName(host);
        DatagramSocket sock = new DatagramSocket(  );
        
        // Allocate the data buffer
        byte[] buffer = new byte[PACKET_SIZE];

        // The udp packet we will send and receive
        DatagramPacket packet = new DatagramPacket(
            buffer, PACKET_SIZE, servAddr, DAYTIME_PORT);

        /* Send empty max-length (-1 for null byte) packet to server */
        packet.setLength(PACKET_SIZE-1);
        sock.send(packet);
        Debug.println("net", "Sent request");

        // Receive a packet and print it.
        sock.receive(packet);
        Debug.println("net", "Got packet of size " + packet.getLength(  ));
        System.out.print("Date on " + host + " is " + 
            new String(buffer, 0, packet.getLength(  )));
    }
}

$ jikes +E -d . DaytimeUDP.java
$ java DaytimeUDP darian
Date on darian is Sat Jan 27 12:42:41 2001
$