Networking and Sockets

1. Networking and Sockets IST 411 Lecture 2 Spring 2004

2. Protocols • A protocol are the rules that govern information communication • HTTP (HyperText Markup Language) – protocol to interpret HTML pages • SMTP (Simple Mail Transfer Protocol) – rules that govern transfer of e-mail • FTP (File Transfer Protocol) – rules for the transfer of files across the Internet • Each is an example of an application protocol

3. Protocols • URL (Uniform Resource Locator) – web address that specifies 3 bits of information: • Method used to transfer information, i.e., HTTP or FTP • Address of the host computer, i.e., www.hbg.psu.edu • Path describing where the file is located on the host, i.e., /hbg/course00.html

4. Protocols • E-mail address is specified by the SMTP protocol which includes: • A local mailbox address, i.e., jxs121 • Followed by the address of the computer, i.e., psu.edu

5. Protocols • DNS (Domain Name System) – governs how names can be translated into numeric addresses, i.e., psu.edu • DNS divides the Internet into a hierarchy of domains and subdomains • Domains are names like com, edu, mil • DNS servers – job to translate names to numeric addresses whenever they are requested to do so by clients such as the SMTP or HTTP server

6. Client/Server Applications • HTTP, FTP, SMTP, and DNS protocols are examples of client/server protocols • Client/Server application is divided into two subtasks: • One performed by the client • One performed by the server Request service Server Client Provide service

7. Protocols • Internet has several levels of software and hardware, each with collection of protocols • Application level is the highest level • Transmission Protocols represent the next level • TCP (Transfer Control Protocol) • UDP (User Datagram Protocol) • Govern the transfer of large blocks of information called packets between networked computers

8. Protocols • Lowest end of the hierarchy of protocols are those governing transmission of bits or electronic pulses over wires • Most of these protocols are built into the hardware • On top of these are protocols such as ethernet protocol and token ring protocol • These lower level protocols are vastly different from each other

9. Protocols • To connect disparate networks, use IP (Internetworking Protocol) to translate one network protocol to a common format

10. Client/Server Applications • Java includes a powerful set of classes the supports network programming and handles even the highest level of protocols needed to write client/server applications • java.net package

12. Client/Server Applications • java.net.URL class • Used to download WWW pages • Used to access files stored on the Web as input files to an applet or application program • Socket and ServerSocket classes • Methods that let us develop our own networking applications • Enable read and write data through InputStreams and OutputStreams

13. Client/Server Applications • DatagramPacket and DatagramSocket classes • Provide support for lower level networking applications based on Internet packets

14. Client/Server Applications • Lab 1 – Look up local host

15. Client/Server Applications • Algorithm to download a data file from the Internet: • Create a URL instance • Open an InputStream to it • Read the data • Close the stream

16. Client/Server Applications try { URL url = new URL(fileURL); data = url.openStream(); String line = data.readLine(); data.close(); } catch (MalformedURLException e) { System.out.println("ERROR: " + e.getMessage()); } catch (IOException e) { System.out.println("ERROR: " + e.getMessage()); }

17. Client/Server Applications • Lab 2 – Real estate lookup

18. Sockets

20. Sockets • Client / server model based on socket connection • Socket – simple communication channel for two programs to communicate over a network • Two-way communication • Server creates a certain port and waits until a client requests a connection • Port is a particular address or entry point on the host computer represented by an integer

21. Sockets • By convention, port numbers from 0 to 1023 are reserved for special services

22. Sockets • Ports above 1023 may be used for any user application • The port is not a hardware concept • It is a network abstraction to allow easy development programs reading and writing data to a network • Each communication channel into and out of a TCP/IP computer is identified by: • IP address • Port number • Combination creates the abstraction called a socket

23. Sockets • Socket has two channels • one for input • one for output

24. Server Protocol • Four steps to create the server: • Create a ServerSocket and establish a port number (ServerSocket constructor throws an IOException and a BindException) • Listen for and accept a connection from a client • Converse with the client • Close the socket

25. Server Protocol try { ServerSocket server = new ServerSocket(8000); Socket nextClient = server.accept( ); // Communicate with the client nextClient.close( ); } catch (IOException ioe) { System.out.println("I/O error: " + ioe.getMessage()); }

26. Let’s Practice • The Server.

27. Client Protocol • Three steps to create the client: • Open a socket connection to the server, giving its address (Socket constructor throws an IOException) • Converse with the server • Close the connection

28. Client Protocol String hostname = "localhost"; // Could be a URL try { Socket timeDay = new Socket (hostname, 8000); // Communicate with the client timeDay.close(); } catch (IOException e) { System.out.println("*** ERROR: " + e.getMessage()); }

29. Let’s Practice • The Client.