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
  • The World Wide Web
  • Email

Physical Media

• Network cabling
  • Electrical impulses represent the information flow
• Telephone wires using a modem
  • Data travels encoded as tones
• Air in a wireless network
  • 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
  • consists of the information one computer wants to send to another
• Network protocol data
  • describes how to reach the intended computer
  • describes how to check for errors in the transmission

Network Protocol

• Local area network protocols
  • Microsoft Networking
  • Novell NetWare
  • AppleTalk
• Protocol for connecting local area networks
  • 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
  • Each is one byte (a number between 0 and 255)
  • For example 130.65.86.66
• To send data to B, A needs to know B's Internet address
  • 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
  • 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:
  • It asks the DNS for the numeric Internet Address
  • 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
  • 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
  • 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
  • In this case: java.sun.com
  • This identifies the computer to which you want to connect
  • 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
  • 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

Browser Steps

• The browser displays the contents of the file for you to see
• Since this file is an HTML file, the browser decodes the HTML codes into fonts, paragraphs etc.
• If the file contains images, the browser makes more GET requests through the same connection

Telnet

• Telnet program allows you to
  • Type characters to send to a remote computer and
  • 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

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:
  • The string received by the client equals QUIT command
  • Or it is null
  • 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.java

File BankService.java

File BankClient.java

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 Commands

command
request properties
blank line

• HTTP command
  • Such as GET item HTTP/1.0
• request properties
  • Such as If-Modified-Since
• blank line
  • 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 Response

status line containing response code 
response parameters
blank line

• status line containing response code
  • HTTP/1.1 200 OK
  • HTTP/1.1 400 NOT FOUND
• response parameters
  • There may be several lines of parameters
• blank line
  • 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.java

Dynamic Content

• Content of many pages changes by the minute
  • Stock prices
  • Weather
• These pages are not stored in files
• When you request such a page
  • The web server starts the appropriate program
  • Supplies it with input from the request
  • Sends the output back

Request for Web Page with Dynamic Content

• The web server knows this URL denotes a program
  • /server
  • /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
  • GET
  • 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/1.0
  blank 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
  • Spaces are encoded as + character
  • 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
  • With a single name cytstzip
  • And value of either a ZIP code or a city and state
• Run the program from the command line
  • java PostZipQuery Beverly Hills, CA

File PostZipQuery.java