Networking in Python2025 (programs allll)

Important method for socket programming

Application Layer 2-4
Socket programming
goal: learn how to build client/server applications that
communicate using sockets
socket: dropbox between application process and end-end-
transport protocol
Internet
controlled
by OS
controlled by
app developer
transport
application
physical
link
network
process
transport
application
physical
link
network
process
socket

Socket programming
Two socket types for two transport services:
– UDP: unreliable datagram
– TCP: reliable, byte stream-oriented
Application Example:
1. Client reads a line of characters (data) from its
keyboard and sends the data to the server.
2. The server receives the data and converts
characters to uppercase.
3. The server sends the modified data to the client.
4. The client receives the modified data and displays
the line on its screen.

Socket programming with UDP
UDP: no “connection” between client & server
• no handshaking before sending data
• sender explicitly attaches IP destination address and port # to
each packet
• rcvr extracts sender IP address and port# from received
packet
UDP: transmitted data may be lost or received
out-of-order
Application viewpoint:
• UDP provides unreliable transfer of groups of bytes
(“datagrams”) between client and server

Client/server socket interaction: UDP
close
clientSocket
read datagram from
clientSocket
create socket:
clientSocket =
DatagramSocket()
Create datagram with server IP and
port=x; send datagram via
clientSocket
create socket, port= x:
serverSocket =
DatagramSocket(x)
read datagram from
serverSocket
write reply to
serverSocket
specifying
client address,
port number
Application 2-7
server (running on serverIP) client

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
addr using DNS
Example: Java client (UDP)

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 datagram
to server
read datagram
from server
Example: Java client (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
Example: Java server (UDP)

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
write out
datagram
to socket
end of while loop,
loop back and wait for
another datagram
create datagram
to send to client
Example: Java server (UDP)

Example app: UDP client
import socket
serverName = ‘hostname’
serverPort = 12000
clientSocket = socket.socket(socket.AF_INET,
socket.SOCK_DGRAM)
message = raw_input(’Input lowercase sentence:’)
clientSocket.sendto(message,(serverName, serverPort))
modifiedMessage, serverAddress =
clientSocket.recvfrom(2048)
print modifiedMessage
clientSocket.close()
Python UDPClient
include Python’s socket
library
create UDP socket for server
get user keyboard
input
Attach server name, port to
message; send into socket
print out received string and
close socket
read reply characters from
socket into string

Example app: UDP server
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET, SOCK_DGRAM)
serverSocket.bind(('', serverPort))
print “The server is ready to receive”
while 1:
message, clientAddress = serverSocket.recvfrom(2048)
modifiedMessage = message.upper()
serverSocket.sendto(modifiedMessage, clientAddress)
Python UDPServer
create UDP socket
bind socket to local port
number 12000
loop forever
Read from UDP socket into
message, getting client’s
address (client IP and port)
send upper case string back
to this client

Socket programming with TCP
client must contact server
• server must be first running
• server must have created
socket (dropbox) that
welcomes client’s contact
client connects to server by:
• creating TCP socket,
specifying IP address, port
number of server process
• client socket is now bound
to that specific server
• server accepts connect by:
– creating new connection-
specific socket
– allows server to talk with
multiple clients
application viewpoint:
TCP provides reliable, in-order
byte-stream transfer (“pipe”)
between client and server

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

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
clientSocket object
of type Socket,
connect to server
create
output stream
attached to socket
this package defines Socket()
and ServerSocket() classes
server port #
server name,
e.g., www.umass.edu

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();
}
}
create
input stream
attached to socket
send line
to server
read line
from server
close socket
(clean up behind yourself!)
Example: Java client (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()));
wait, on welcoming
socket accept() method
for client contact create,
new socket on return
create
welcoming socket
at port 6789
create input
stream, attached
to socket
Example: Java server (TCP)

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

Example app:TCP client
import socket
serverName = ‘servername’
serverPort = 12000
clientSocket = socket.socket(socket.AF_INET,
socket.SOCK_STREAM)
clientSocket.connect((serverName,serverPort))
sentence = raw_input(‘Input lowercase sentence:’)
clientSocket.send(sentence)
modifiedSentence = clientSocket.recv(1024)
print ‘From Server:’, modifiedSentence
clientSocket.close()
Python TCPClient
create TCP socket for server,
remote port 12000
No need to attach server
name, port

Example app:TCP server
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET,SOCK_STREAM)
serverSocket.bind((‘’,serverPort))
serverSocket.listen(1)
print ‘The server is ready to receive’
while 1:
connectionSocket, addr = serverSocket.accept()
sentence = connectionSocket.recv(1024)
capitalizedSentence = sentence.upper()
connectionSocket.send(capitalizedSentence)
connectionSocket.close()
Python TCPServer
create TCP welcoming
socket
server begins listening for
incoming TCP requests
loop forever
server waits on accept()
for incoming requests, new
socket created on return
read bytes from socket (but
not address as in UDP)
close connection to this client
(but not welcoming socket)

A python program to know ip address of the
website

Networking in Python2025 (programs allll)

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