Networking in Java Representation and Management of Data on the Internet

Client-Server Model Port 80 Server application Client application

Clients Client - initiates connection –retrieves data –displays data –responds to user input –requests more data Examples: –Web Browser –Chat Program –PC accessing files

Servers Server - responds to connection –receives request for data –looks it up –delivers it Examples: –Web Server –Database Server –Domain Name Server

Networking Basics Communication layers: Link (device, driver,…) Network (IP) Transport (TCP, UDF) Our ApplicationsApplication (HTTP, FTP, Telnet)

Internet Architecture Model Application Layer DATA Transport Layer HEADERDATA Internet Layer HEADER DATA Network Layer HEADER DATA

TCP and UDP TCP (Transmission Control Protocol) –connection-based protocol –provides a reliable flow of data between two computers UDP (User Datagram Protocol) –a protocol that sends independent packets of data, called datagrams, from one computer to another –arrival of datagrams is not gaurantees –UDP is not connection-based like TCP

How to Choose TCP or UDP Use TCP when reliability is critical: –HTTP –FTP –Telnet Use UDP when reliability is not critical: –Ping –Clock –Audio and Video transmission –NFS – Network File System

Host and Port Destination in the Internet is identified by host + port –a 32 bits IP-address –a 16 bits port Q: Why don’t we specify the port in a Web browser? Ports are restricted –Do not use them

Destination of a Connection Q: How does an HTTP request ‘knows’ where is the server to which it is intended? Q: How does an HTTP response ‘knows’ where is the server to which it is intended? Q: There can be more then one application (ever some servers) running on the same host, how do we know to which application a message is intended?

A Connection to Many Applications Q: There can be many applications that are on the same time connected to the same host, (for example, many browser and one search engine) How do we send the right transmission to each client? A: By adding the IP address of the client and the port of the client to the IP packets

Known Ports Some known ports are –20, 21: FTP –23: telnet –25: SMTP –43: whois –80: HTTP –119: NNTP Client Application web browser mail client

Internet Addresses InetAddress – a final class that represents Internet Protocol (IP) addresses and the names of hosts Getting the InetAdddress –getLocalHost Returns the local host –getByName(String host) For the given host name one of its IP addresses is returned –getAllByName(String host) For a given host name all its IP addresses are returned

Methods of InetAddress getHostAddress –Returns the IP address of the host –The address is in the form “%d.%d.%d.%d” getHostName –Returns the name of the host

Protocol Host Name Port Number File Name Reference Working with URLs URL (Uniform Resource Locator) - a reference (an address) to a resource on the Internet

Creating URLs The class URL is defined in the package java.net Basic constructor: URL w3c = new URL(" Relative links: –Are created from baseURL + relativeURL URL amaya = new URL(w3c, “Amaya/Amaya.html”); URL jigsaw = new URL(w3c, “Jigsaw/#Getting”);

Creating URLs (cont.) The following two are equivalent: URL dbiNotes = new URL(“ + ”~dbi/main.html#notes”); URL dbiNotes = new URL(“http”, “ 80, “~dbi/main.html#notes”); Construction of URLs can throw MalformedURLException

Why Do We Need the URL Class? The main usage of URL is for parsing URLs –getting the protocol –getting the host –getting the port –getting the file name –getting the reference An example of parsing a URL

Running the Example

Java URLInfo Protocol: http Host: File name: Port: -1 Reference: null Java URLInfo Protocol: http Host: File name: /~dbi/main.html Port: 80 Reference: notes

Reading From A URL url InputStreamReader openStream BufferedReader readLine Page Content

Reading From a URL Example of reading from a given URL: UrlReader.java For reading a URL using a proxy, we do the following: Java –Dhttp.proxyHost=wwwproxy.cs.huji.ac.il –Dhttp.proxyPort=8080 UrlReader url

Reading … Java UrlReader url The content of the file

Connecting to A URL url InputStreamReader getInputStream BufferedReader URLConnection openConnection PrintWriter getOutputStream Page println readLine

Interacting with a CGI script 1.Create a URL 2.Open a connection to the URL 3.Set output capability on the URLConnection 4.Get an output stream from the connection This output stream is connected to the standard input stream of the cgi-bin script on the server 5.Write to the output stream 6.Close the output stream

HTTP connections You can create a connection to an HTTP server with an object HttpURLConnection This object extends the URLConnection object –getResponseCode –getResponseMessage –setRequestMethod Look in the Java API HTTP/ OK … HTTP/ OK …

URLEncoder contains a utility method encode for converting a string into an encoded format To convert a string, each character is examined in turn: –The ASCII characters ' a ' – ' z ', ' A ' – ' Z ', ' 0 ' – ' 9 ', ".", "-", "*", "_" remain the same –Space is converted into a plus sign ' + ' –All other characters are converted into the 3-character string " %xy ", where xy is the two-digit hexadecimal representation of the lower 8-bits of the character

URL Connection Example The next example connects to a CGI script on – a search tool is given a word to search SearchWalla

Sockets Communication is between sockets A socket is one end-point of a two-way communication link between two programs running on the network A socket has a binding to a port to which it listens A socket implementation gives us the ability to read from it and write to it as if it is a file

In Sockets A socket ‘knows’ the followings –The port number of the remote host –The host name (InetAddress) of the remote host –The local port to which it is bound –The local address of its local host

Sockets The server has a socket that listen to a known port, e.g., Web server and port 80 The server waits (listening to the socket) till a client requests a connection A client requests a connection on the known host and port of the server Q: What should the server do so that it can serve other clients as well?

Client Requests for a Connection Port 80 Server application Client application Port 8090 Port 8888 Port: 8090 Local Port: 80 Local host: Remote host: Port: 8888 Local Port: 80 Local host: Remote host:

Socket Class Socket – implements the client side of the connection Class ServerSocket – implements the server side of the connection

Using a Socket InputStreamReader getInputStream BufferedReader readLine Page Content Socket host port

Using a Socket // Constructors (partial list) public Socket() public Socket(InetAddress address, int port); public Socket(String host, int port); // Methods (partial list) public void close(); public InetAddress getInetAddress(); public int getLocalPort(); public InputStream getInputStream(); public OutputStream getOutputStream(); public int getPort(); public String toString();

Using a Socket (client) A client: 1.Opens a socket (with a binding to which port?) 2.Opens an input stream and output stream to the socket 3.Reads from and write to the stream according to the client’s protocol 4.Closes the streams 5.Closes the socket

Using a Socket (cont.) A server: 1.Open a socket 2.Open an input stream and output stream to the socket 3.Read from and write to the stream according to the server's protocol 4.Close the streams 5.Close the socket

A Client Example The following is a client that connects to a Time Server (port 13) and returns the curret time TimeClient

Running the TimeClient java TimeClient Tue Mar 13 12:42:

ServerSocket // Constructors (partial list) public ServerSocket(int port); public ServerSocket(int port, int count); // Methods (partial list) public Socket accept(); public void close(); public InetAddress getInetAddress(); public int getLocalPort(); public String toString(); What happens when a running program reaches accept()? What happens when a running program reaches accept()? The size or requests queue with default of 50 requests Why don’t we have getPort in addition to getLocalPort? Why don’t we have getPort in addition to getLocalPort?

A Server Example The following is a time server that returns the time: TimeServer Note: When the ServerSocket constructor is given port number 0, a random free port is chosen

More on Server Socket A ServerSocket waits for requests to come in over the network It performs some operation based on that request, and then possibly returns a result to the requester The actual work of the ServerSocket is performed by an instance of the SocketImpl class The abstract class SocketImpl is a common superclass of all classes that actually implement sockets It is used to create both client and server sockets

Server side import java.net.*; import java.io.*; // A server that says 'hello' class HelloServer { public static void main(String[] args) { int port = Integer.parseInt(args[0]); ServerSocket server = null; try { server = new ServerSocket(port); } catch (IOException ioe) { System.err.println("Couldn't run"+ "server on port "+port); return; }

Server side while(true) { try { Socket connection = server.accept(); BufferedReader reader = new BufferedReader( new InputStreamReader( connection.getInputStream())); PrintWriter writer = new PrintWriter( new OutputStreamWriter( connection.getOutputStream())); String clientName = reader.readLine(); writer.println("Hello "+clientName); writer.flush(); } catch (IOException ioe1) {} }

Client side import java.net.*; import java.io.*; // A client of an HelloServer class HelloClient { public static void main(String[] args) { String hostname = args[0]; int port = Integer.parseInt(args[1]); Socket connection = null; try { connection = new Socket(hostname, port); } catch (IOException ioe) { System.err.println("Connection failed"); return; }

Client side try { BufferedReader reader = new BufferedReader( new InputStreamReader( connection.getInputStream())); PrintWriter writer = new PrintWriter( new OutputStreamWriter( connection.getOutputStream())); writer.println(args[2]); // client name String reply = reader.readLine(); System.out.println("Server reply: "+reply); writer.flush(); } catch (IOException ioe1) { }

Datagrams Datagram packets are used to implement a connectionless, packet based, delivery service Each message is routed from one machine to another based solely on information contained within that packet Multiple packets sent from one machine to another might be routed differently, and might arrive in any order Packets may be lost or duplicated during transit The class DatagramPacket represents a datagram in Java

DatagramPacket Class //Constructors public DatagramPacket(byte ibuf[], int ilength); public DatagramPacket( byte ibuf[], int ilength, InetAddress iaddr, int iport); // Methods public synchronized InetAddress getAddress(); public synchronized int getPort(); public synchornized byte[] getData(); int getLength(); void setAddress(InetAddress iaddr); void setPort(int iport); void setData(byte ibuf[]); void setLength(int ilength);

DatagramSocket This class represents a socket for sending and receiving datagram packets Addressing information for outgoing packets is contained in the packet header A socket that is used to read incoming packets must be bound to an address ( sockets that are used for sending must be bound as well, but in most cases it is done automatically ) There is no special datagram server socket class Since packets can be lost, the ability to set timeouts is important

Class DatagramSocket // Constructors DatagramSocket() DatagramSocket(int port) DatagramSocket(int port, InetAddress iaddr) // Methods void close() InetAddress getLocalAddress() int getLocalPort() int getSoTimeout() void receive(DatagramPacket p) void send(DatagramPacket p) setSoTimeout(int timeout)

Echo Servers A common network service is an echo server An echo server simply sends packets back to the sender A client creates a packet, sends it to the server, and waits for a response Echo services can be used to test network connectivity and performance

Echo Client Example import java.net.*; import java.io.*; import java.util.*; public class EchoClient { static int echoPort = 7; static int msgLen = 16; static int timeOut=1000; public static void main(String argv[]) { try { DatagramSocket socket = new DatagramSocket(); DatagramPacket packet; byte msg[] = new byte[msgLen]; InetAddress echoHost = InetAddress.getByName(argv[0]); packet = new DatagramPacket(msg,msgLen,echoHost,echoPort); socket.send(packet); socket.setSoTimeout(timeOut); socket.receive(packet); } catch (InterruptedIOException e) {System.out.println("Timeout");} catch (Exception e) {}}}

EchoServer import java.net.*;import java.io.*;import java.util.*; public class EchoServer { static int echoPort = 7000; static int msgLen = 1024; public static void main(String args[]) { try { DatagramSocket socket = new DatagramSocket(echoPort); DatagramPacket p,reply; byte msg[] = new byte[msgLen]; packet = new DatagramPacket(msg,msgLen); for (;;) { sock.receive(p); System.out.println(p.getAddress()); reply = new DatagramPacket(p.getData(),p.getLength(),p.getAddress(),p.getPort()); socket.send(reply); } } catch (Exception e) {} }}

Java Net Classes ClassDescription DatagramPacketThis class represents a datagram packet. DatagramSocketThis class represents a socket for sending and receiving datagram packets. InetAddressThis class represents an Internet Protocol (IP) address. MulticastSocketThe multicast datagram socket class is useful for sending and receiving IP multicast packets. ServerSocketThis class implements server sockets. Socket This class implements client sockets (also called just "sockets"). URLA pointer to a "resource" on the World Wide Web. URLConnectionThe superclass of all classes that represent a communications link between an application and a URL.