1. Network Programming and Java Sockets
Vijayalakshmi R

2. Agenda Introduction Elements of Client Server Computing
Networking Basics
Understanding Ports and Sockets
Java Sockets
Implementing a Server
Implementing a Client
Sample Examples
Conclusions

3. 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.

4. Internet Applications Serving Local and Remote Users
PC client
Internet Server
Local Area Network
PDA

5. Internet & Web as a delivery Vehicle

6. 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.

7. Elements of C-S Computing
a client, a server, and network
Request
Client
Server
Network
Result
Client machine
Server machine

8. Networking Basics Applications Layer Transport Layer Network 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,..)

9. 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,..)

10. 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,..)

11. Understanding Ports
The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer.
server
P o r t
TCP
Client
app
app
app
app
port
port
port
port
TCP or UDP
Packet
Data
port#
data

12. 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

13. 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.

14. 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.
server
Connection request
port
Client

15. 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
port
port
Client
port
Connection

16. 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

17. Java Sockets ServerSocket(1234) Output/write stream Input/read stream
Client
Input/read stream
Socket(“ ”, 1234)
It can be host_name like “mandroo.cs.mu.oz.au”

18. Socket programming
Goal: learn how to build client/server application that communicate using sockets
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

19. 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
controlled by
application
developer
controlled by
application
developer
process
TCP with
buffers,
variables
socket
process
TCP with
buffers,
variables
socket
controlled by
operating
system
controlled by
operating
system
internet
host or
server
host or
server

20. 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

21. Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
TCP
connection setup
close
connectionSocket
read reply from
clientSocket
create socket,
connect to hostid, port=x
clientSocket =
Socket()
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
send request using
clientSocket
read request from
connectionSocket
write reply to

22. 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.
Client
process
client TCP socket

23. 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)

24. 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());
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket

25. Example: Java client (TCP), cont.
Create
input stream
attached to socket
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(); }
Send line
to server
Read line
from server

26. 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()));
Create
welcoming socket
at port 6789
Wait, on welcoming
socket for contact
by client
Create input
stream, attached
to socket

27. Example: Java server (TCP), cont
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence); }
Create output
stream, attached
to socket
Read in line
from socket
Write out line
to socket
End of while loop,
loop back and wait for
another client connection

28. 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

29. Client/server socket interaction: UDP
Server (running on hostid)
create socket,
clientSocket =
DatagramSocket()
Client
Create, address (hostid, port=x,
send datagram request
using clientSocket
create socket,
port=x, for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
close
clientSocket
read reply from
clientSocket
write reply to
serverSocket
specifying client
host address,
port number

30. Example: Java client (UDP)
process
Input: receives packet (recall thatTCP received “byte stream”)
Output: sends packet (recall
that TCP sent “byte stream”)
client UDP socket

31. 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();
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS

32. Example: Java client (UDP), cont.
Create datagram with data-to-send,
length, IP addr, port
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(); }
Send datagram
to server
Read datagram
from server

33. 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);
Create
datagram socket
at port 9876
Create space for
received datagram
Receive
datagram

34. 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 addr
port #, of
sender
Create datagram
to send to client
Write out
datagram
to socket
End of while loop,
loop back and wait for
another datagram