Basics of Socket Programming using python

goal: learn how to build client/server applications that
communicate using sockets
socket: door 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

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
Socket Programming w/ UDP

close
clientSocket
read datagram from
clientSocket
create socket:
clientSocket =
socket(AF_INET,SOCK_DGRAM)
Create datagram with server IP and
port=x; send datagram via
clientSocket
create socket, port= x:
serverSocket =
socket(AF_INET,SOCK_DGRAM)
read datagram from
serverSocket
write reply to
serverSocket
specifying
client address,
port number
server (running on serverIP) client

from socket import *
serverName = ‘hostname’
serverPort = 12000
clientSocket = 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

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

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 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 that particular client
• allows server to talk with
multiple clients
• source port numbers used
to distinguish clients (more
in Chap 3)
TCP provides reliable, in-order
byte-stream transfer (“pipe”)
between client and server
application viewpoint:
Socket Programming w/ TCP

wait for incoming
connection request
connectionSocket =
serverSocket.accept()
create socket,
port=x, for incoming
request:
serverSocket = socket()
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

serverName = ’servername’
serverPort = 12000
clientSocket = socket(AF_INET, 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

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)

MultiClient TCP programming in Python

Multithreaded TCP Server in Python

Multithreaded TCP Client in Python

Basics of Socket Programming using python

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