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Chapter 22. Internet Networking. CHAPTER GOALS. To understand the concept of sockets To learn how to send and receive data through sockets To implement network clients and servers To communicate with web servers and server-side applications through Hypertext Transfer Protocol (HTTP). - PowerPoint PPT Presentation
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Chapter 22
Internet Networking
CHAPTER GOALS• To understand the concept of sockets • To learn how to send and receive data
through sockets • To implement network clients and servers • To communicate with web servers and
server-side applications through Hypertext Transfer Protocol (HTTP)
Internet • A worldwide collection of networks, routing
equipment, and computers • Uses a common set of protocols to define how the
parties will interact with each other • Many different services are offered over the Internet
o The World Wide Web o Email
Physical Media• Network cabling
o Electrical impulses represent the information flow
• Telephone wires using a modem o Data travels encoded as tones
• Air in a wireless network o Signals are sent by modulating radio frequencies
Data Transmission
• Data transmission consists of sending and receiving streams of zeros and ones along the network connection
Two Types of Information • Application data
o consists of the information one computer wants to send to another
• Network protocol data o describes how to reach the intended computer o describes how to check for errors in the transmission
Network Protocol• Local area network protocols
o Microsoft Networking o Novell NetWare o AppleTalk
• Protocol for connecting local area networks o Internet Protocol (IP)
Sending Data from A to B across the Internet
• A is you home computer • It is connected by phone lines to an Internet Service Provider (ISP) • The ISP is connected to an Internet Access Point • B is on an local area network at XYZ Computers. • XYZ has its own Internet Access Point • The Internet Access Points are connected by a complex
collection of pathways (the Internet) • Over these pathways a message sent from one access point can eventually reach any access point
Two Computers Communicating across the Internet
Destination Address• Data must be marked with a destination address • In IP, addresses are denoted by a sequence of four
numbers o Each is one byte (a number between 0 and 255) o For example 130.65.86.66
• To send data to B, A needs to know B's Internet address o A includes that address in the protocol portion when sending the data
Domain Naming Service• In addition to an IP address, computers can have
an easy-to-remenber domain name o For example, java.sun.com
• A service called a Domain Naming Service (DNS) translates from domain name to Internet address
• When A wants to request data from a domain name: o It asks the DNS for the numeric Internet
Address o It includes the numeric address with the request
for data
Packets
• IP breaks large chunks of data up into more manageable packets
• Each packet is delivered separately • Each packet in a larger transmission may
be sent by a different route. • Packets are numbered • The recipient reassembles the data
Transmission Control Protocol• Internet Protocol (IP) does not notify the sender if data is lost or garbled
• This is the job of a higher level protocol Transmission Control Protocol (TCP)
• The most commonly used Internet services use TCP with IP (TCP/IP)
TCP's Job • Attempt to deliver the data
• Try again if there are failures
• Notify the sender whether or not the attempt was successful
Port Numbers
• One computer can offer multiple services over the Internet
o For example, both a web server program and an email server program
• When data are sent to that computer, they need to indicate which program is to receive the data
• IP uses port numbers for this
Port Numbers
• A port number is an integer between 0 and 65,535
• The sending program must know the port number of the receiving program
• This number is included in the transmitted data
Contents of TCP Packet•The Internet address of the recipient
•The port number of the recipient
• Internet address of the sender
•The port number of the sender
The OSI Reference Model
Application Level Protocol
• TCP/IP mechanism establishes an Internet connection between two ports on two computers.
• Each Internet application has its own application protocol
• This application protocol describes how data for that application are transmitted
Hypertext Transfer Protocol• The application protocol used by the World Wide Web is Hypertext Transfer Protocol (HTTP) • A web address is called a Uniform Resource Locator (URL) • You type a URL into the address window of your browser
o For example, http://java.sun.com/index.html
Browser Steps• You type http://java.sun.com/index.html into the browser's address • The browser examines the part of the URL between the double slash and the first single slash
o In this case: java.sun.com o This identifies the computer to which you
want to connect o Because it contains letters, this part of the
URL is a domain name • The browser sends a quest to a DNS server to obtain the Internet address of the computer with the domain name java.sun.com
Browser Steps• From the http: prefix to the URL, the browser
decides that the protocol you want to use is HTTP
o HTTP uses port 80 by default • The browser establishes a TCP/IP connection to
port 80 at the Internet address determined above • The browser deduces from the /index.html that
you want to see the file /index.html • It sends this request formatted as an HTTP
command through the established connection GET /index.html HTTP/1.0
a blank line
Browser Steps• The web server running on the computer whose Internet Address was obtained above receives the request
• It decodes the request
• It fetches the file /index.html
• It sends the file back to the browser
Telnet• Telnet program allows you to o Type characters to send to a remote
computer and o View the characters that the remote
computer sends back • It is a useful tool to establish test connections
with servers • You can imitate the browser connection by
using a dialog box or typing at the command line telnet java.sun.com 80
Telnet • After Telnet starts, type the following without using backspace
GET /index.html HTTP/1.0 then hit enter twice
• The server responds to the request with the file
• Telnet is not a browser
• It does not understand HTML tags so it just displays everything it was sent
Web Server Response in Telnet
HTTP Commands
Client Program - Sockets
• A socket is an object that encapsulates a TCP/IP connection •There is a socket on both ends of a connection •Syntax to create a socket in a Java program
Socket s = new Socket(hostname, portnumber); •Code to connect to the HTTP port of server, java.sun.com final int HTTP_PORT = 80;
Socket s = new Socket("java.sun.com", HTTP_PORT); •If it can't find the host, the Socket constructor throws an UnknownHostException
Client Program - Input and Output Streams
• Use the input and output streams attached to the socket s to communicate with the other endpoint of the connection • Code to obtain the input and output streams
InputStream in = s.getInputStream();OutputStream out = s.getOutputStream();
• When you send data to the output stream out, the socket forwards them to the server • The socket catches the server's response and you can read it through the input stream in • When you are done communicating with the server, close the socket s.close();
Client and Server Sockets
Client Program - Readers and Writers
• InputStream and OutputStream send and receive bytes • To send and receive text, get a reader and a writer
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
PrintWriter writer = new PrintWriter(out);
• A PrintWriter buffers the characters and only sends when the buffer is full
• When sending a command, you want the whole command to be sent now so flush the buffer manually
writer.print(command); writer.flush();
Client Program - WebGet
• This program lets you retrieve any item from a web server
• You specify the host and item from the command line
• For example: java WebGet java.sun.com index.html
File WebGet.java 01: import java.io.BufferedReader;02: import java.io.InputStream;03: import java.io.InputStreamReader;04: import java.io.IOException;05: import java.io.OutputStream;06: import java.io.PrintWriter;07: import java.net.Socket;08: 09: /**10: This program demonstrates how to use a socket to communicate11: with a web server. Supply the name of the host and the12: resource on the command-line, for example13: java WebGet java.sun.com index.html14: */15: public class WebGet16: {17: public static void main(String[] args) throws IOException
18: {19: // get command-line arguments20: 21: if (args.length != 2)22: {23: System.out.println("usage: java WebGet host resource");24: System.exit(0);25: }26: String host = args[0];27: String resource = args[1];28: 29: // open socket30: 31: final int HTTP_PORT = 80;32: Socket s = new Socket(host, HTTP_PORT);33: 34: // get streams35: 36: InputStream in = s.getInputStream();37: OutputStream out = s.getOutputStream();
38: 39: // turn streams into readers and writers40: 41: BufferedReader reader = new BufferedReader(42: new InputStreamReader(in));43: PrintWriter writer = new PrintWriter(out); 44: 45: // send command46: 47: String command = "GET /" + resource + " HTTP/1.0\n\n";48: writer.print(command);49: writer.flush();50: 51: // read server response52: 53: boolean done = false;54: while (!done)55: {56: String input = reader.readLine();57: if (input == null) done = true;
58: else System.out.println(input);59: }60: 61: // always close the socket at the end62: 63: s.close(); 64: }65: }
A Server Program• This is a server program that enable clients to manage a set of bank accounts in a bank
• When you develop a server application, you need some application-level protocol
• The client can use this protocol to interact with the server
• A simple bank access protocol is described on the next slide
Simple Bank Access Protocol
A Server Program• The server waits for the client to connect on a certain port. We choose 8888
• To listen for incoming connections, use a server socket
• To construct a server socket, provide the port number ServerSocket server = new ServerSocket(8888);
• Use the accept method to wait for client connection and obtain a socket Socket s = server.accept();
Server Program - Readers and Writers
• Get the input and output streams connected to the socket s
• Create a reader and writer BufferedReader in = new BufferedReader( new
InputStreamReader(s.getInputStream())); PrintWriter out = new PrintWriter(
s.getOutputStream());
• The server reads a command from the client String line = in.readLine();
Server Program • The socket s is closed if: o The string received by the client
equals QUIT command
o Or it is null
o A null string means the client has disconnected
Server Program • Use a StringTokenizer to analyze
the client command string
• The first token is the command
• The second token is the account number
Server Program
• If the command is DEPOSIT, deposit to the given account • If the command is WITHDRAW, withdraw from the given account • Then send the account number and new balance to the client out.println(n + " " + bank.getBalance(account));
Server Program
• The server program never stops • When you are done running the
sever, kill it.
To Try Out Server Program • Run the server
• Use Telnet to connect to localhost , port number 8888
• Start typing commands
Using the Telnet Program to Connect to the BankServer
File BankServer.java01: import java.io.IOException;02: import java.net.ServerSocket;03: import java.net.Socket;04: 05: /**06: A server that executes the Simple Bank Access Protocol.07: */08: public class BankServer09: { 10: public static void main(String[] args ) throws IOException11: { 12: final int ACCOUNTS_LENGTH = 10;13: Bank bank = new Bank(ACCOUNTS_LENGTH);14: final int SBAP_PORT = 8888;15: ServerSocket server = new ServerSocket(SBAP_PORT);16: System.out.println("Waiting for clients to connect...");
17:
18: while (true)19: {20: Socket s = server.accept();21: 22: BankService service = new BankService(s, bank);23: service.doService();24: s.close();25: }26: }27: }28:29:30:31:32:33:34:35:
File BankService.java01: import java.io.BufferedReader;02: import java.io.InputStream;03: import java.io.InputStreamReader;04: import java.io.IOException;05: import java.io.OutputStream;06: import java.io.PrintWriter;07: import java.net.Socket;08: import java.util.StringTokenizer;09: 10: /**11: Executes Simple Bank Access Protocol commands12: from a socket.13: */14: public class BankService15: {
16: /**17: Constructs a service object that processes commands18: from a socket for a bank.19: @param aSocket the socket20: @param aBank the bank21: */22: public BankService(Socket aSocket, Bank aBank)23: {24: s = aSocket;25: bank = aBank;26: }27: 28: /**29: Executes all commands until the QUIT command or the30: end of input.31: */32: public void doService() throws IOException33: {34: BufferedReader in = new BufferedReader(35: new InputStreamReader(s.getInputStream()));
36: PrintWriter out = new PrintWriter(37: s.getOutputStream());38: 39: while (true)40: { 41: String line = in.readLine();42: System.out.println("Received: " + line);43: if (line == null || line.equals("QUIT")) 44: return;45: 46: String response = executeCommand(line);47: 48: System.out.println("Sending: " + response);49: out.println(response);50: out.flush();51: }52: }53: 54: /**55: Executes a single command.
56: @param line the command57: @return the reply to send to the client.58: */59: public String executeCommand(String line)60: {61: StringTokenizer tokenizer 62: = new StringTokenizer(line);63: String command = tokenizer.nextToken();64: int account = Integer.parseInt(tokenizer.nextToken());65: if (command.equals("DEPOSIT"))66: {67: double amount = Double.parseDouble(68: tokenizer.nextToken());69: bank.deposit(account, amount);70: }71: else if (command.equals("WITHDRAW"))72: {73: double amount = Double.parseDouble(74: tokenizer.nextToken());75: bank.withdraw(account, amount);
76: } 77: else if (!command.equals("BALANCE"))78: return "Invalid command";79: 80: return account + " " + bank.getBalance(account);81: }82: 83: private Socket s;84: private Bank bank;85: }
File BankClient.java01: import java.io.BufferedReader;02: import java.io.InputStream;03: import java.io.InputStreamReader;04: import java.io.IOException;05: import java.io.OutputStream;06: import java.io.PrintWriter;07: import java.net.Socket;08: 09: /**10: This program tests the bank server.11: */12: public class BankClient13: {14: public static void main(String[] args) throws IOException15: {16: final int SBAP_PORT = 8888;17: Socket s = new Socket("localhost", SBAP_PORT);
18: InputStream in = s.getInputStream();19: OutputStream out = s.getOutputStream();20: BufferedReader reader = new BufferedReader(21: new InputStreamReader(in));22: PrintWriter writer = new PrintWriter(out); 23: 24: String command = "DEPOSIT 3 1000\n";25: System.out.print("Sending: " + command);26: writer.print(command);27: writer.flush();28: String response = reader.readLine();29: System.out.println("Receiving: " + response);30: 31: command = "WITHDRAW 3 500\n";32: System.out.print("Sending: " + command);33: writer.print(command);34: writer.flush();35: response = reader.readLine();36: System.out.println("Receiving: " + response);
37: 38: command = "QUIT\n";39: System.out.print("Sending: " + command);40: writer.print(command);41: writer.flush();42: 43: s.close();44: }45: }46: 47: 48: 49:50:
URLConnection Class • Provides convenient support for HTTP
• Can also handle FTP (file transfer protocol)
• Takes care of socket connection for you
• Makes it easy to communicate with a web server without giving HTTP commands
URL Connections• Construct a URL object from a URL starting with the http or
ftp prefix URL u = new URL("http://java.sun.com/index.html");
• Use the URL's openConnection() method to get the URLConnection URLConnection connection = u.openConnection();
• Call the getInputStream method to obtain an input stream InputStream in = connection.getInputStream();
• Construct a reader from the stream BufferedReader in = new BufferedReader(new
InputStreamReader(in));
URL Connections
• Use the reader to read a line at a time from the resource
boolean done = false; while (!done) {
String input = reader.readLine(); if (input == null) done = true;
else do something with the input }
HTTP Commandscommand request properties blank line • HTTP command
o Such as GET item HTTP/1.0 • request properties
o Such as If-Modified-Since • blank line
o separates the command and its request properties from the input data
URLConnection Class• Has methods to set request properties
connection.setIf ModifiedSince(date);
• Set the request properties before calling getInputStream
• The URLConnection class sends all the request properties that are set to the web server
Server Responsestatus line containing response code response parameters
blank line • status line containing response code
o HTTP/1.1 200 OK o HTTP/1.1 400 NOT FOUND
• response parameters o There may be several lines of parameters
• blank line o separates the status and response parameters from
the requested data
Retrieving Response Code and Message
• Cast the URLConnection object to the HttpConnection subclass
• Get the response code with getResponseCode
• Get the response message with getResponseMessage
Retrieve Other Response Information from URLConnection
• Content-Type int length = connection.getContentLength();
• Content-Content String type = connection.getContentType();
File URLGet.java01: import java.io.BufferedReader;02: import java.io.InputStream;03: import java.io.InputStreamReader;04: import java.io.IOException;05: import java.io.OutputStream;06: import java.io.PrintWriter;07: import java.net.HttpURLConnection;08: import java.net.URL;09: import java.net.URLConnection;10: 11: /**12: This program demonstrates how to use an URL connection 13: to communicate with a web server. Supply the URL on the14: command-line, for example15: java UrlGet http://java.sun.com/index.html16: */17: public class URLGet
18: {19: public static void main(String[] args) throws IOException20: {21: // get command-line arguments22: 23: if (args.length != 1)24: {25: System.out.println("usage: java UrlGet URL");26: System.exit(0);27: }28: 29: // open connection30: 31: URL u = new URL(args[0]);32: URLConnection connection = u.openConnection();33: 34: // check if response code is HTTP_OK (200)35: 36: HttpURLConnection httpConnection = (HttpURLConnection)connection;37: int code = httpConnection.getResponseCode();38: if (code != HttpURLConnection.HTTP_OK)
39: {40: String message = httpConnection.getResponseMessage(); 41: System.out.println(code + " " + message);42: return;43: }44: 45: // read server response46: 47: InputStream in = connection.getInputStream();48: BufferedReader reader = new BufferedReader(49: new InputStreamReader(in));50: 51: boolean done = false;52: while (!done)53: {54: String input = reader.readLine();55: if (input == null) done = true;56: else System.out.println(input);57: }58: }59: }
Dynamic Content• Content of many pages changes by the minute
o Stock prices o Weather
• These pages are not stored in files
• When you request such a page o The web server starts the appropriate
program o Supplies it with input from the request o Sends the output back
Request for Web Page with Dynamic Content
• The web server knows this URL denotes a program o /server o /cgi-bin
• The web server starts the appropriate program
• Supplies it with input from the request • Sends the program's output back to the client
Form Data• There are two HTTP commands for
supplying input to server-side programs o GET o POST
• GET is simpler to use but can only be used for short inputs
GET• Input is appended after the program URL following a ?
• Input is usually name/value pairs
• Example: GET Mach.usno.navy.mil/cgi-bin/aa_moonphases?year=2000 HTTP/1.0blank line
• Multiple name/value pairs are separated by an ampersand(&)city=Chicago&zip=60614
Encoding• GET and Post requests must both be encoded
• The scheme is called URL encoding
• Characters that are not ASCII letters or numbers are encoded o Spaces are encoded as + character o Other bytes are encoded as %xy where xy is the
hexadecimal value of the byte
• Use static methods URLEncoder.encode and URLDecoder.decode
POST• Use POST for longer inputs
• Format POST url Content-Type type Content-Length length other request headers blank line input data
• Most common source of POST request is the submission of HTML form data
POST
• Don't format the POST request yourself
• Use the URLConnection class
POST• Call setDoOutput method of URLConnection
• Call getOutputStream
• Send the data to that stream
• You can send URL encoded name/value pairs
• Close the output stream when you are done
PostZipQuery.java• The Program makes a ZIP code lookup by
calling a server-side program hosted by US Postal Service
• The program expect POST input of name/value format
o With a single name cytstzip o And value of either a ZIP code or a city and
state • Run the program from the command line o java PostZipQuery Beverly Hills, CA
File PostZipQuery.java01: import java.io.BufferedReader;02: import java.io.InputStream;03: import java.io.InputStreamReader;04: import java.io.IOException;05: import java.io.OutputStream;06: import java.io.PrintWriter;07: import java.net.URL;08: import java.net.URLConnection;09: import java.net.URLEncoder;10: 11: /**12: This program posts a query to a United States Postal Service13: server that can look up the ZIP code for a city name. Supply14: the city name and an optional state on the command line, 15: such as 16: java PostZipQuery Los Angeles, CA17: */
18: public class PostZipQuery19: { 20: public static void main(String[] args) throws IOException21: { 22: // concatenate all command line arguments23: 24: String input;25: if (args.length > 0)26: {27: input = args[0];28: for (int i = 1; i < args.length; i++)29: input += " " + args[i];30: }31: else32: input = "90210";33: 34: // establish URL connection35: 36: URL u = new URL(37: "http://www.usps.gov/cgi-bin/zip4/ctystzip");
38: URLConnection connection = u.openConnection();39: 40: // send POST data41: 42: connection.setDoOutput(true);43: OutputStream out = connection.getOutputStream();44: PrintWriter writer = new PrintWriter(out);45: writer.print("ctystzip=" 46: + URLEncoder.encode(input) + "\n");47: writer.close();48: 49: // print server response50: 51: InputStream in = connection.getInputStream();52: BufferedReader reader = new BufferedReader(new53: InputStreamReader(in));54: 55: boolean done = false;56: while (!done)57: {
58: String inputLine = reader.readLine();59: if (inputLine == null)60: done = true;61: else62: System.out.println(inputLine);63: }64: reader.close();65: }66: }