Datagram Programming

1. Datagram Programming • A datagram is an independent, self-contained message sent over the network whose arrival, arrival time, and content are not guaranteed. • Connectionless • It need a socket • It need a package • It is more efficient for some application which does not require the reliable, point-to-point channel provided by TCP

2. DatagramPacket class • The DatagramPacket class represents a datagram packet. Datagram packets are used to implement a connectionless packet delivery service. • Each message is routed from one machine to another based solely on information contained within the packet. • Packet delivery is not guaranteed

3. Java.net.DatagramPacket int length InetAddress address int port DatagramPacket(byte[] buf, int length, InetAddress host, int port) DatagramPacket(byte[] buf, int length) byte[] getData() void setData(byte[] buf)

4. DatagramSocket class • DatagramSocket class represents a socket for sending and receiving datagram packets. • A datagram socket is the sending or receiving point for a packet delivery service. • Each packet sent or received on a datagram socket is individually addressed and routed • Multiple packets sent from one machine to another may be routed differently, and may arrive in any order.

5. Create a server datagram • To create a server DatagramSocket use the constructor DatagramScoket(int port), which binds the socket with the specified port on the local host machine. • To create a client DatagramSocket, use the constructor DatagramSocket(), which binds the socket with any available port on the local host machine. • To send data, you need to create a packet, fill in the contents, specify the Internet address and port number for the receiver and invoke the send(packet) method on a DatagramSocket • To receive data, you need to create an empty packet and invoke the receive (packet) method on a DatagramSocket.

6. DatagramServer • DatagramSocket socket; • socket = new DatagramSocket(8000); • DatagramPacket recv = new DatagramPacket( buf, buf.length); • socket.receive(recv); • Get data from buf or from recv.getData()

7. DatagramClient • DatagramSocket socket; • socket = new DatagramSocket(); • byte[] buf = new byte[256]; • InetAddress add = new InetAddress(serverName); • DatagramPacket sd = new DatagramPacket(buf, buf.length, add, 8000); • // fill in the contents in buf • socket.send(sd); • DatagramPacket recv = new DatagramPacket(buf, buf.length); • socket.receive(recv);

8. notes • Since datagram are connectionless, a DatagramPacket can be sent to multiple clients, and multiple clients can receive a packet from a same server. • The server creates a DatagramSocket on port 8000. No DatagramSocket can be created again on the same port • The client creates a DatagramSocket on an available port. The port number is dynamically assigned to the socket. • You can launch multiple clients simultaneously and each client’s datagram will be different

9. Convert data for sending • Read data as a string (str)from a text field • Str.trim() will return a string(trim_str) with blank characters trimmed on both sides • Trim_str.getBytes() will store the string in array of Bytes and return it • Then create the packet with the byte [] • Then call socket.setData( packet)

10. Extract data from receiving • Recv = new DatagramPacket(buf, buf.length); • socket.receive(recv); • Convert to a string • String str = new String(buf); • str.trim(); • Convert to numerical value • Double x = Double.parseDouble(str);