Introduction to Sockets

1. Introduction to Sockets • “A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application to which data is destined to be sent.”

2. Sockets • Interface between application and protocol software • de facto standard • Usually, part of OS itself • Integrated with system I/O. Like file I/O. • open-read-write-close paradigm • Referenced by integer socket descriptor

4. Networking Review • TCP/IP Protocol Suite • Application programming interface to TCP/IP (API) • BSD Unix • Java.net.Socket • Platform Independence

6. Ports • TCP and UDP use ports to map incoming data to a particular process running on the computer • Ports are 16 bit numbers • Need IP Address (32-bits) and port. • 0-1023 Restricted out of 65,535.

7. Networking Classes in Java • TCP: URL, URLConnection, Socket, ServerSocket • UDP: DatagramPacket, DatagramSocket, MulticastSocket • Socket and ServerSocket form two ends of communicaiton link.

9. Procedure • Open a socket • Open an input and output stream to the socket • Read from and write to the stream according to the server’s protocol • close streams • close socket

10. Client/Server • Request - Response • A server application waits passively for contact. • A client application initiates communication actively.

11. Clients • Client temporarily when remote access needed. Does other computation locally. • Runs locally • Invoked by user. • Initiates contact • Can access multiple services as needed • No special h/w or OS

12. Servers • Special purpose to provide one service • Invoked automatically at boot • Runs on server (shared computer) • Waits on connections passively • Accept connections from arbitrary clients • Requires powerful h/w and special purpose OS

13. Datagrams • An independent self-contained message • Not Reliable • DatagramPacket and DatagramSocket • DatagramSocket: Specify port or not • DatagramPacket: Contains addressing information • MulticastSocket: To broadcast

14. import java.io.*; import java.net.*; import java.util.*; public class QuoteMain { public static void main(String[] args) throws IOException { BufferedReader in = null; boolean moreQuotes = true; DatagramSocket socket = new DatagramSocket(4445); try { in = new BufferedReader(new FileReader("one-liners.txt")); } catch (FileNotFoundException e) { System.err.println("Could not open quote file. Serving time instead."); } }

15. while (moreQuotes) { try { byte[] buf = new byte[256]; // receive request DatagramPacket packet = new DatagramPacket(buf, buf.length); socket.receive(packet); // figure out response String dString = null; if (in == null) dString = new Date().toString(); else { try { if ((dString= in.readLine()) == null) { in.close(); moreQuotes = false; dString = "No more quotes. Goodbye."; } } catch (IOException e) { dString = "IOException occurred in server."; } }

16. buf = dString.getBytes(); // send the response to the client at "address" and "port" InetAddress address = packet.getAddress(); int port = packet.getPort(); packet = new DatagramPacket(buf, buf.length, address, port); socket.send(packet); } catch (IOException e) { e.printStackTrace(); moreQuotes = false; } } socket.close(); }

17. import java.io.*; import java.net.*; import java.util.*; public class QuoteClient { public static void main(String[] args) throws IOException { if (args.length != 1) { System.out.println("Usage: java QuoteClient <hostname>"); return; } // get a datagram socket DatagramSocket socket = new DatagramSocket(); // send request byte[] buf = new byte[256]; InetAddress address = InetAddress.getByName(args[0]); DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4445); socket.send(packet);

18. // get response packet = new DatagramPacket(buf, buf.length); socket.receive(packet); // display response String received = new String(packet.getData()); System.out.println("Quote of the Moment: " + received); socket.close(); } }