1.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(1/11) Java Sockets and Simple Networking Joel Adams and Jeremy Frens Calvin College

2.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(2/11) The TCP/IP Stack TCP/IP is a layered protocol family: Hardware  Application layer protocols convert app. program data to/from messages FTPTELNETSMTP... messages  Transport layer protocols convert messages to/from packets. TCPUDP packets  The Internet layer protocol (IP) converts packets to/from IP datagrams IP datagrams  The Network interface layer protocol (X.25) converts IP datagrams to/from network’s frames to send/receive... X.25 frames TCP/IP Applications data

3.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(3/11) host j host i TCP/IP app i TCP/IP app j Application Transport Internet Network Application Transport Internet Network network End-to-End Communication The stacks enable any two hosts to communicate in a point-to-point fashion. A TCP/IP network is a collection of communication links and hosts, with each host having a TCP/IP stack. Control info is also sent between specific layers... Each layer of the stack (at each end) performs its respective task. data messagepacketsdatagrams frames

4.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(4/11) host j host i network The Client-Server Model Most TCP/IP applications use the client-server model, splitting a network application into two parts: Servers for specific services use well-known ports: echo7 daytime13 quote17 ftp-data20 ftp-cntl21 telnet23 smtp25 dns53 finger79  A client that contacts a server to access the service it offers: client app  An always-running server (or daemon) that provides a specific service: server app

5.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(5/11) Sockets  To provide a service to clients, a server:  creates a socket using a given protocol (TCP or UDP) and port  waits for a client to send a request to the socket (blocking)  receives a request via the socket, processes it  sends any results back to the client via the socket  To request service from a server, a client:  creates a socket  uses it to send a request for service to a server  uses it to receive any results from the server Most TCP/IP applications communicate using an abstraction called a socket, that represents a communication endpoint. Sockets are the software infrastructure of TCP/IP networks.

6.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(6/11) Java Sockets  ServerSocket is a class representing TCP server sockets,  Socket is a class representing TCP client sockets, and  DatagramSocket is a class representing UDP sockets, that provides send() and receive() methods by which clients and servers can send/receive DatagramPacket objects. Java provides separate classes to represent various kinds of sockets, including: Java also provides class InetAddress and other useful classes in its java.net package.

7.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(7/11) TCP Echo Client A client for the echo service is easy to write... import java.net.*; // Socket import java.io.*; // BufferedReader, PrintWriter,... class EchoClient { public static void main(String [] args) { //... Omitted: Code to input echo server’s name and port try { BufferedReader keyboard = new BufferedReader( new InputStreamReader( System.in ) ); PrintWriter screen = new PrintWriter( new OutputStreamWriter( System.out ), true ); // … continued on next slide …

8.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(8/11) TCP Echo Client (ii) //... continued from previous slide … Socket s = new Socket(InetAddress.getByName(remoteHost), remotePort); BufferedReader sin = new BufferedReader( new InputStreamReader( s.getInputStream() )); PrintWriter sout = new PrintWriter( new OutputStreamWriter( s.getOutputStream() ), true ); String lineFromUser = null, lineFromServer = null; for (;;) { lineFromUser = keyboard.readLine(); if ( lineFromUser.equals("quit") ) break; sout.println(lineFromUser); lineFromServer = sin.readLine(); screen.println(lineFromServer); } s.close(); } catch( Exception e ) { System.err.println(e); } } Note: This program does not follow the MVC pattern…

9.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(9/11) TCP Echo Client A client using the MVC pattern is also easy to write... import java.net.*; // Socket import java.io.*; // BufferedReader, PrintWriter,... class EchoClient2 { public static void main(String [] args) { //... Omitted: Code to input echo server’s name and port try { BufferedReader keyboard = new BufferedReader( new InputStreamReader( System.in ) ); PrintWriter screen = new PrintWriter( new OutputStreamWriter( System.out ), true ); // … continued on next slide … So far this is identical to the previous version…

10.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(10/11) TCP Echo Client (ii) //... continued from previous slide … EchoClient theClient = new EchoClient(remoteHost, remotePort); String lineFromUser = null, lineFromServer = null; for (;;) { lineFromUser = keyboard.readLine(); if ( lineFromUser.equals("quit") ) break; theClient.send(lineFromUser); lineFromServer = theClient.receive(); screen.println(lineFromServer); } However, This version isolates the core functionality of the client in a separate EchoClient class, and so the rest is much simpler than the previous version: The EchoClient class provides a constructor, plus simple send() and receive() methods…

11.  2003 Joel C. Adams. All Rights Reserved. Calvin CollegeDept of Computer Science(11/11) Exercise: Part II Use the remaining time to work through part II of today’s exercise. We will be demo-ing your Part II solutions at the end of today’s session!