Socket programming

JAVA Socket Programming

2003.3.19
Joonbok Lee
KAIST

What is a socket?
• Socket
– The combination of an IP address and a port number. (RFC 793
,original TCP specification)
– The name of the Berkeley-derived application programming
interfaces (APIs) for applications using TCP/IP protocols.
– Two types
• Stream socket : reliable two-way connected communication streams
• Datagram socket

• Socket pair
– Specified the two end points that uniquely identifies each TCP
connection in an internet.
– 4-tuple: (client IP address, client port number, server IP address,
server port number)

Client-server applications
• Implementation of a protocol standard defined in an RFC. (FTP,
HTTP, SMTP…)
– Conform to the rules dictated by the RFC.
– Should use the port number associated with the protocol.

• Proprietary client-server application.
– A single developer( or team) creates both client and server
program.
– The developer has complete control.
– Must be careful not to use one of the well-known port number
defined in the RFCs.

* well-known port number : managed by the Internet Assigned
Numbers Authority(IANA)

Socket Programming with TCP

Figure 2.6-1: Processes communicating through TCP
sockets

The application developer has the ability to fix a few TCP
parameters, such as maximum buffer and maximum segment sizes.

Sockets for server and client
• Server
– Welcoming socket
• Welcomes some initial contact from a client.
– Connection socket
• Is created at initial contact of client.
• New socket that is dedicated to the particular client.

• Client
– Client socket
• Initiate a TCP connection to the server by creating a socket
object. (Three-way handshake)
• Specify the address of the server process, namely, the IP
address of the server and the port number of the process.

Socket functional calls
• socket (): Create a socket
• bind(): bind a socket to a local IP address and port #
• listen(): passively waiting for connections
• connect(): initiating connection to another socket
• accept(): accept a new connection
• Write(): write data to a socket
• Read(): read data from a socket
• sendto(): send a datagram to another UDP socket
• recvfrom(): read a datagram from a UDP socket
• close(): close a socket (tear down the connection)

Sockets

Figure 2.6-2: Client socket, welcoming socket and connection
socket

Socket-programming using TCP
TCP service: reliable byte stream transfer
socket( )
bind( ) server
socket( ) listen( )
client bind( )
connect( ) TCP conn. request
accept( )
send( ) TCP ACK
recv( )

recv( )
send( )
close( ) close( )

controlled by
application process
process
developer socket
socket
controlled by TCP with
TCP with
operating buffers, internet
system buffers,
variables
variables

Socket programming with TCP
keyboard monitor

Example client-server app:
• client reads line from standard

inFromUser
input
input (inFromUser stream) , stream

sends to server via socket Client
Process Input stream:
(outToServer stream) process sequence of bytes
• server reads line from socket output stream: into process
• server converts line to sequence of bytes
uppercase, sends back to client out of process

inFromServer
outToServer
• client reads, prints modified output
stream
input
stream

line from socket
(inFromServer stream) client TCP
clientSocket
socket TCP
socket

to network from network

Client/server socket interaction: TCP
Server (running on hostid) Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()

TCP create socket,
wait for incoming
connection request connection setup connect to hostid, port=x
connectionSocket = clientSocket =
welcomeSocket.accept() Socket()

send request using
read request from clientSocket
connectionSocket

write reply to
connectionSocket read reply from
clientSocket
close
connectionSocket close
clientSocket

JAVA TCP Sockets
• In Package java.net
– java.net.Socket
• Implements client sockets (also called just “sockets”).
• An endpoint for communication between two machines.
• Constructor and Methods
– Socket(String host, int port): Creates a stream socket and connects it to the
specified port number on the named host.
– InputStream getInputStream()
– OutputStream getOutputStream()
– close()

– java.net.ServerSocket
• Implements server sockets.
• Waits for requests to come in over the network.
• Performs some operation based on the request.
– ServerSocket(int port)
– Socket Accept(): Listens for a connection to be made to this socket and
accepts it. This method blocks until a connection is made.

TCPClient.java
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());

TCPClient.java
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();

}
}

TCPServer.java
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()));

TCPServer.java

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

clientSentence = inFromClient.readLine();

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

outToClient.writeBytes(capitalizedSentence);

}
}
}

Socket Programming with
UDP
• UDP
– Connectionless and unreliable service.
– There isn’t an initial handshaking phase.
– Doesn’t have a pipe.
– transmitted data may be received out of order, or lost

• Socket Programming with UDP
– No need for a welcoming socket.
– No streams are attached to the sockets.
– the sending hosts creates “packets” by attaching the IP destination
address and port number to each batch of bytes.
– The receiving process must unravel to received packet to obtain the
packet’s information bytes.

Client/server socket interaction: UDP
Server (running on hostid) Client

create socket,
port=x, for create socket,
clientSocket =
incoming request: DatagramSocket()
serverSocket =
DatagramSocket()
Create, address (hostid, port=x,
send datagram request
using clientSocket
read request from
serverSocket

write reply to
serverSocket
specifying client read reply from
host address, clientSocket
port umber close
clientSocket

Example: Java client (UDP)
keyboard monitor

inFromUser
input
stream

Client
Process
Input: receives
process
packet (TCP
received “byte
Output: sends
stream”)
packet (TCP sent

receivePacket
sendPacket
“byte stream”) UDP UDP
packet packet

client UDP
clientSocket
socket UDP
socket

to network from network

JAVA UDP Sockets
• In Package java.net
– java.net.DatagramSocket
• A socket for sending and receiving datagram
packets.
– DatagramSocket(int port): Constructs a datagram
socket and binds it to the specified port on the local
host machine.
– void receive( DatagramPacket p)
– void send( DatagramPacket p)
– void close()

UDPClient.java
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();

UDPClient.java
      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();

    }
}

UDPServer.java
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);

          String sentence = new String(receivePacket.getData());

UDPServer.java

   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);

      }
}
}

Building a Simple Web Server
• Handles only one HTTP request
• Accepts and parses the HTTP request
• Gets the required file from the server’s
file system.
• Creates an HTTP response message
consisting of the requested file
preceded by header lines
• Sends the response directly to the client

WebServer.java
import java.io.*;
import java.net.*;
import java.util.*;
class WebServer{
    public static void main(String argv[]) throws Exception {
          String requestMessageLine;
     String fileName;
          ServerSocket listenSocket = new ServerSocket(6789);
     Socket connectionSocket = listenSocket.accept();

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

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

WebServer.java
requestMessageLine = inFromClient.readLine();

StringTokenizer tokenizedLine =
new StringTokenizer(requestMessageLine);

if (tokenizedLine.nextToken().equals("GET")){
          fileName = tokenizedLine.nextToken();
          if (fileName.startsWith("/") == true )
              fileName = fileName.substring(1);

File file = new File(fileName);
      int numOfBytes = (int) file.length();
          FileInputStream inFile = new FileInputStream (fileName);
           byte[] fileInBytes = new byte[numOfBytes];

inFile.read(fileInBytes);

WebServer.java
     outToClient.writeBytes("HTTP/1.0 200 Document Followsrn");

if (fileName.endsWith(".jpg"))
     outToClient.writeBytes("Content-Type: image/jpegrn");

if (fileName.endsWith(".gif"))
     outToClient.writeBytes("Content-Type: image/gifrn");

outToClient.writeBytes("Content-Length: " + numOfBytes + "rn");

outToClient.writeBytes("rn");
outToClient.write(fileInBytes, 0, numOfBytes);
     connectionSocket.close();
}
    else System.out.println("Bad Request Message");
}
}

Concurrent server
• Servers need to handle a new
connection request while processing
previous requests.
– Most TCP servers are designed to be
concurrent.
• When a new connection request arrives
at a server, the server accepts and
invokes a new process to handle the
new client.

How to handle the port numbers
cosmos% netstat –a –n –f inet
Active Internet connections (including servers)
Proto Recv-Q Send-Q Local Address Foreign Address (state)
tcp 0 0 *.23 *.* LISTEN

tcp 0 0 192.249.24.2.23 192.249.24.31.1029 ESTABLISHED
tcp 0 0 *.23 *.* LISTEN

tcp 0 0 192.249.24.2.23 192.249.24.31.1029 ESTABLISHED
tcp 0 0 192.249.24.2.23 192.249.24.31.1030 ESTABLISHED
tcp 0 0 *.23 *.* LISTEN

Socket programming: references

C-language tutorial (audio/slides):
• “Unix Network Programming” (J. Kurose),
http://manic.cs.umass.edu/~amldemo/courseware/intro.html

Java-tutorials:
• “All About Sockets” (Sun tutorial),
http://www.javaworld.com/javaworld/jw-12-1996/jw-12-
sockets.html
• “Socket Programming in Java: a tutorial,”
http://www.javaworld.com/javaworld/jw-12-1996/jw-12-
sockets.html

Socket programming

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