Learning Outcomes. Networking. Sockets. TCP/IP Networks. Hostnames and DNS TCP/IP

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1 CP4044 Lecture 7 1 Networking Learning Outcomes To understand basic network terminology To be able to communicate using Telnet To be aware of some common network services To be able to implement client and server programs in Java using the following classes: Sockets Server sockets URL connections CP4044 Lecture 7 2 Clients TCP/IP Networks Network Server TCP/IP networks are based on a packetswitched, client/server model A server runs continuously, listening for requests from clients A client initiates a transaction with the server CP4044 Lecture 7 3 Sockets Common, basic communications mechanism. A bi-directional serial communication channel that associates two network connected computers. Acts like a pipe that allows data flow (via a stream) between the computers. As far as the programmer is concerned: Data is written to a socket in exactly the same way as it would be written to a file or display. It can then be read from the other end of the socket in exactly the same way as data is read from a file or keyboard. The intricacies of communications are handled within the socket class. As a Java programmer you can get by without knowing much about networking. CP4044 Lecture 7 4 TCP/IP IP (Internet Protocol) routes data to the destination Each connected to the network has a unique IP address, e.g TCP (Transmission Control Protocol) looks after the transmission of data. Provides a reliable, connection-oriented transport Delivers the data to a process identified by a port number A socket encapsulates the IP address of the destination the Port number of a process running on that CP4044 Lecture 7 5 Hostnames and DNS The host name is the name of the server. In a form that is easily remembered/used by humans e.g. The IP protocol actually needs an Internet address. e.g Host names are translated into IP addresses by DNS, the Domain Name System. Why have two ways of referring to the same host? Allows the host to be moved to a different only DNS entries need to be changed users unaffected Humans don't like using strings of numbers Computers like numbers allows easy implementation of hierarchies, subnets etc. CP4044 Lecture 7 6

2 CP4044 Lecture 7 7 Ports Ports allow s to provide multiple services. A server process listens on a specific (published) port for requests. Ports are identified by a -bit integer. Ports up to 24 are reserved for specific services Ports above are free for use. Used by clients, they are known as ephemeral ports and are allocated numbers starting at working down. Some Well-Known Port Numbers Port Name RFC Purpose Daytime 867 Gets the date and time 21 FTP 959 Transfers files 23 Telnet 854 Log on to a remote system 25 SMTP 821 Internet mail service 70 Gopher 36 Document archives 80 http 1945 Transfers Web resources 1 Pop Download Internet mail RFCs are obtained from CP4044 Lecture 7 8 Server A server process runs in a loop, continually listening for connection requests via its server socket. Once a connection request has been received the server process can then read data from the socket and process it. It then resumes listening for the next request. A common model is for the server process to create a new process to service the request, so that it can immediately resume listening for the next connection request. CP4044 Lecture 7 9 Client A client process attempts to create a socket connection to a remote server, usually in response to some sort of user initialised event. Client/server communication is similar to a telephone conversation: Somebody (the client) picks up a phone and dials a number. Another phone rings and somebody (the server) picks it up. A bi-directional conversation then continues until somebody hangs up. The phone number called is similar to an IP address. Ports are similar to different extension numbers. CP4044 Lecture 7 Telnet The Telnet command line tool uses the Telnet protocol ( based on TCP/IP, to allow a computer to act as a remote terminal on another anywhere on the Internet. It allows us to experiment with client/server systems. command prompt server port C:\> telnet clun.scit.wlv.ac.uk 80 CP4044 Lecture 7 11 World Wide Web Service HTTP Port 80 Web servers listen on port 80 for connections Once connected the client can send a number of simple commands. e.g. GET followed by the name of a resource. We can retrieve raw HTML using a Telnet client C:\> telnet 80 Trying Connected to clun.scit.wlv.ac.uk. Escape character is '^]'. GET <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" " g/tr/html4/loose.dtd"> <html lang="en"> <head> <meta http-equiv="content-type" content="text/html; charset=iso "> <title>school of Computing and IT</title> CP4044 Lecture 7

3 CP4044 Lecture 7 File Transfer Protocol Service FTP Port 21 http :// FTP servers listen on port 21 for connections USER and PASS commands are required before other commands can be issued. C:\> telnet ftp.scit.wlv.ac.uk 21 Trying Connected to clun.scit.wlv.ac.uk. Escape character is '^]'. 220 clun FTP server (Version 4.3 Mon Oct :20: BST 2000) ready. USER anonymous 331 Guest login ok, send address as password. PASS mel.ralph@wlv.ac.uk... Output omitted... QUIT 230- Goodbye Connection to host lost Network Programming with Java Java provides many classes for programming in a TCP/IP network environment These are in the java.net package Writing client/server programs which implement sockets, use these classes: java.net.serversocket java.net.socket A number of higher-level classes are available. We look at just one: java.net.url CP4044 Lecture 7 ServerSocket Class import java.net.*; Constructor: ServerSocket(int port) Methods include: Socket accept() void close() Use this class only for Server programs. As with all I/O, sockets can throw a variety of exceptions. CP4044 Lecture 7 Socket Class Constructor: Socket (String host, int port) Some methods: InputStream getinputstream() OutputStream getoutputstream() void setsotimeout(int timeout) void close() CP4044 Lecture import java.io.*; import java.net.*; A Simple Server public class Server { public static void main(string[] args) { String messagein, messageout; ServerSocket ssock = new ServerSocket(6789); while (true) { Socket connsock = ssock.accept(); BufferedReader innet = new BufferedReader( new InputStreamReader( connsock.getinputstream() )); DataOutputStream outnet = new DataOutputStream(connsock.getOutputStream()); messagein = innet.readline(); messageout = messagein.touppercase() + "\n"; outnet.writebytes(messageout); catch(ioexception e) { e.getmessage() CP4044 Lecture 7 Simple Server Line 7 The main method may throw exceptions. It requires an exception handler. Line 8 Constructs a ServerSocket object to listen on port Lines 9-21 Infinite loop to continually process requests. Line Waits (by blocking the thread) for a connection to be made to the ServerSocket. Once connected, the connsock socket can be used to communicate with the client. CP4044 Lecture 7

4 CP4044 Lecture 7 19 Simple Server Lines 11- Constructs a BuffererReader to read text data line-by-line from the client Lines - Constructs an OutputDataStream to send data to the client. Lines -20 Read a line of text from the client, convert it to upper case and send it back to the client import java.io.*; A Simple Client import java.net.*; ublic class Client { public static void main(string[] args) { BufferedReader kbd = new BufferedReader( new InputStreamReader(System.in)); Socket csock = new Socket("localhost", 6789); DataOutputStream outnet = new DataOutputStream(csock.getOutputStream()); BufferedReader innet = new BufferedReader( new InputStreamReader(csock.getInputStream())); String message = kbd.readline(); outnet.writebytes(message + "\n"); message = innet.readline(); System.out.println("Server sent: " + message); csock.close(); CP4044 Lecture 7 20 Simple Client Line 5 Exceptions may be thrown, so use exception handlers. Lines 6-7 Construct a BufferedReader to read lines of text from the keyboard. Lines 9- Construct a DataOutputStream to send data through the socket. (Note it is necessary to write raw bytes to the socket). Simple Client Line A line of text is read from the keyboard into the message variable. Line The text stored in the message variable is output through the socket. Line A line of text is read in through the socket. Lines 11- Construct a BufferedReader to read in lines of text through the socket. If binary data was required DataInputStream could have been used (to process primitive data types) or bytes could be read directly from the InputStream. CP4044 Lecture 7 21 Line The text that was read from the socket is printed out. Line The socket is closed, making it unavailable and freeing up resources. CP4044 Lecture 7 22 Testing Client/Server Progs When program testing, it is convenient to run both the server and the client on the same The local can be addressed with this reserved address: IP address: Hostname: localhost Run the Server first in one command prompt Then run the Client in a second command prompt Running Programs across the Network Run the Server and Client programs on separate networked s You will need to know the IP address of the that the Server is running on: ipconfig (for Windows) ifconfig (for Linux) Use this IP address when creating the socket in the Client program Will not be able to connect if the is running a firewall CP4044 Lecture 7 23 CP4044 Lecture 7 24

5 CP4044 Lecture 7 25 Universal Resource Locator (URL) Identifies a resource on the internet protocol host port If not specified uses the default for the given protocol, e.g. 80 for http path The location of the resource being requested URLs may contain other information such as user names, passwords and locations of resources within resources. See The Java URL class The URL class has a number of uses, including: Validating a url string entered by the user Will throw MalformedURLException if the string does not have valid syntax Parsing a url string Class provides methods to return the different parts of the url string getprotocol(), gethost(), getpath(), getport(), etc. Provides high level means of communication Do not need to create sockets openstream() CP4044 Lecture 7 26 Using the URL Class 01 import java.io.*; The URL class allows simplified 02 import java.net.*; communication for common protocols. 03 public class GetURL { 04 public static void main(string[] args) { 05 String line; int linecount = 0; URL url = new URL(args[0]); InputStream in = url.openstream(); InputStreamReader instr = new InputStreamReader(in); BufferedReader innet = new BufferedReader(inStr); while ((line = innet.readline())!= null) { linecount++; System.out.println(line); System.out.println("Lines read = " + linecount); CP4044 Lecture 7 27 URL Class Example Line 7 The main method may throw exceptions. Line 8 Uses the text that was passed as a parameter to this program to construct a URL object. Lines 9- Opens up an InputStream to read from the URL and attaches it to a BufferedReader to allow line-by-line data retrieval. Lines - Continually reads data from the BufferedReader until it fails to read any more (null is received). CP4044 Lecture 7 28 Validating and Parsing a URL 01 import java.net.*; 02 public class DisplayURL { 03 public static void main(string[] args) { 04 URL url = null; 05 url = new URL(args[0]); catch(malformedurlexception e) { System.out.println("Bad url"); System.exit(1); System.out.println(url); System.out.println("Protocol: "+url.getprotocol()); System.out.println("Path: "+url.getpath()); System.out.println("Host: "+url.gethost()); System.out.println("Port: "+url.getport()); System.out.println("Query: "+url.getquery()); System.out.println("File: "+url.getfile()); CP4044 Lecture 7 29 Summary Introduced some TCP/IP services Telnet - remote connection WWW and FTP services New Java classes introduced: java.net.serversocket java.net.socket java.net.url CP4044 Lecture 7 30