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Network Programming and Java Sockets

Vijayalakshmi R

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Agenda

Introduction Elements of Client Server Computing Networking Basics Understanding Ports and Sockets Java Sockets

Implementing a Server Implementing a Client

Sample Examples Conclusions

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Introduction

Internet and WWW have emerged as global ubiquitous media for communication and changing the way we conduct science, engineering, and commerce.

They also changing the way we learn, live, enjoy, communicate, interact, engage, etc. It appears like the modern life activities are getting completely centered around the Internet.

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Internet Applications Serving Local and Remote Users

Internet Server

PC client

Local Area Network

PDA

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Internet & Web as a delivery Vehicle

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Increased demand for Internet applications

To take advantage of opportunities presented by the Internet, businesses are continuously seeking new and innovative ways and means for offering their services via the Internet.

This created a huge demand for software designers with skills to create new Internet-enabled applications or migrate existing/legacy applications on the Internet platform.

Object-oriented Java technologies—Sockets, threads, RMI, clustering, Web services-- have emerged as leading solutions for creating portable, efficient, and maintainable large and complex Internet applications.

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Network

Reque

st

Result

a client, a server, and network

ClientServer

Client machineServer machine

Elements of C-S Computing

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Networking Basics

Applications Layer Standard apps

HTTP FTP Telnet

User apps Transport Layer

TCP UDP Programming Interface:

Sockets Network Layer

IP Link Layer

Device drivers

TCP/IP Stack

Application

(http,ftp,telnet,…)

Transport

(TCP, UDP,..)

Network

(IP,..)

Link

(device driver,..)

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Networking Basics

TCP (Transport Control Protocol) is a connection-oriented protocol that provides a reliable flow of data between two computers.

Example applications: HTTP FTP Telnet

TCP/IP Stack

Application

(http,ftp,telnet,…)

Transport

(TCP, UDP,..)

Network

(IP,..)

Link

(device driver,..)

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Networking Basics

UDP (User Datagram Protocol) is a protocol that sends independent packets of data, called datagrams, from one computer to another with no guarantees about arrival.

Example applications: Clock server Ping

TCP/IP Stack

Application

(http,ftp,telnet,…)

Transport

(TCP, UDP,..)

Network

(IP,..)

Link

(device driver,..)

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Understanding Ports

The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer.

server

Port

ClientTCP

TCP or UDP

port port port port

app app app app

port# dataData

Packet

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Understanding Ports

Port is represented by a positive (16-bit) integer value

Some ports have been reserved to support common/well known services: ftp 21/tcp telnet 23/tcp smtp 25/tcp login 513/tcp

User level process/services generally use port number value >= 1024

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Sockets

Sockets provide an interface for programming networks at the transport layer.

Network communication using Sockets is very much similar to performing file I/O

In fact, socket handle is treated like file handle. The streams used in file I/O operation are also applicable to

socket-based I/O Socket-based communication is programming language

independent. That means, a socket program written in Java language can

also communicate to a program written in Java or non-Java socket program.

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Socket Communication

A server (program) runs on a specific computer and has a socket that is bound to a specific port. The server waits and listens to the socket for a client to make a connection request.

serverClient

Connection requestport

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Socket Communication

If everything goes well, the server accepts the connection. Upon acceptance, the server gets a new socket bounds to a different port. It needs a new socket (consequently a different port number) so that it can continue to listen to the original socket for connection requests while serving the connected client.

server

ClientConnection

port

port por

t

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Sockets and Java Socket Classes

A socket is an endpoint of a two-way communication link between two programs running on the network.

A socket is bound to a port number so that the TCP layer can identify the application that data destined to be sent.

Java’s .net package provides two classes: Socket – for implementing a client ServerSocket – for implementing a server

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Java SocketsServerSocket(1234)

Socket(“128.250.25.158”, 1234)

Output/write stream

Input/read stream

It can be host_name like “mandroo.cs.mu.oz.au”

Client

Server

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Socket programming

Socket API introduced in BSD4.1 UNIX,

1981 explicitly created, used,

released by apps client/server paradigm two types of transport service

via socket API: unreliable datagram reliable, byte stream-oriented

a host-local, application-created,

OS-controlled interface (a “door”) into which

application process can both send and

receive messages to/from another

application process

socket

Goal: learn how to build client/server application that communicate using sockets

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Socket-programming using TCP

Socket: a door between application process and end-end-transport protocol (UCP or TCP)

TCP service: reliable transfer of bytes from one process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet

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Socket programming with TCP

Client must contact server server process must first be

running server must have created

socket (door) that welcomes client’s contact

Client contacts server by: creating client-local TCP

socket specifying IP address, port

number of server process When client creates socket:

client TCP establishes

connection to server TCP

When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with

multiple clients source port numbers used

to distinguish clients (more in Chap 3)

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint

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Client/server socket interaction: TCP

wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()

create socket,port=x, forincoming request:welcomeSocket =

ServerSocket()

create socket,connect to hostid, port=xclientSocket =

Socket()

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

Server (running on hostid) Client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

22ou

tToS

erve

r

to network from network

inFr

omS

erve

r

inFr

omU

ser

keyboard monitor

Process

clientSocket

inputstream

inputstream

outputstream

TCPsocket

Clientprocess

client TCP socket

Stream jargon

A stream is a sequence of characters that flow into or out of a process.

An input stream is attached to some input source for the process, e.g., keyboard or socket.

An output stream is attached to an output source, e.g., monitor or socket.

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Socket programming with TCP

Example client-server app:1) client reads line from standard input (inFromUser

stream) , sends to server via socket (outToServer stream)

2) server reads line from socket

3) server converts line to uppercase, sends back to client

4) client reads, prints modified line from socket (inFromServer stream)

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Example: Java client (TCP)

import java.io.*; import java.net.*; class TCPClient {

public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence;

BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));

Socket clientSocket = new Socket("hostname", 6789);

DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

Createinput stream

Create client socket,

connect to server

Createoutput stream

attached to socket

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Example: Java client (TCP), cont.

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine();

outToServer.writeBytes(sentence + '\n');

modifiedSentence = inFromServer.readLine();

System.out.println("FROM SERVER: " + modifiedSentence);

clientSocket.close(); } }

Createinput stream

attached to socket

Send lineto server

Read linefrom server

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Example: Java server (TCP)import java.io.*; import java.net.*;

class TCPServer {

public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence;

ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept();

BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));

Createwelcoming socket

at port 6789

Wait, on welcomingsocket for contact

by client

Create inputstream, attached

to socket

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Example: Java server (TCP), cont

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

capitalizedSentence = clientSentence.toUpperCase() + '\n';

outToClient.writeBytes(capitalizedSentence); } } }

Read in linefrom socket

Create outputstream,

attached to socket

Write out lineto socket

End of while loop,loop back and wait foranother client connection

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Socket programming with UDP

UDP: no “connection” between client and server

no handshaking sender explicitly attaches IP

address and port of destination to each packet

server must extract IP address, port of sender from received packet

UDP: transmitted data may be received out of order, or lost

application viewpoint

UDP provides unreliable transfer of groups of bytes (“datagrams”)

between client and server

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Client/server socket interaction: UDP

closeclientSocket

Server (running on hostid)

read reply fromclientSocket

create socket,clientSocket = DatagramSocket()

Client

Create, address (hostid, port=x,send datagram request using clientSocket

create socket,port=x, forincoming request:serverSocket = DatagramSocket()

read request fromserverSocket

write reply toserverSocketspecifying clienthost address,port number

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Example: Java client (UDP)

sendP

ack

et

to network from network

rece

iveP

ack

et

inF

rom

Use

r

keyboard monitor

Process

clientSocket

UDPpacket

inputstream

UDPpacket

UDPsocket

Output: sends packet (recallthat TCP sent “byte stream”)

Input: receives packet (recall thatTCP received “byte stream”)

Clientprocess

client UDP socket

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Example: Java client (UDP)

import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine();

sendData = sentence.getBytes();

Createinput stream

Create client socket

Translate hostname to IP

address using DNS

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Example: Java client (UDP), cont.

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }

}

Create datagram with data-to-send,

length, IP addr, port

Send datagramto server

Read datagramfrom server

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Example: Java server (UDP)

import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

serverSocket.receive(receivePacket);

Createdatagram socket

at port 9876

Create space forreceived datagram

Receivedatagra

m

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Example: Java server (UDP), cont

String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase();

sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }

}

Get IP addrport #, of

sender

Write out datagramto socket

End of while loop,loop back and wait foranother datagram

Create datagramto send to client