User Datagram Client and Server

The user datagram protocol (UDP) works differently from TCP/IP. Where TCP is a stream oriented protocol, ensuring that all of the data is transmitted in the right order, UDP is a message oriented protocol. UDP does not require a long-lived connection, so setting up a UDP socket is a little simpler. On the other hand, UDP messages must fit within a single packet (for IPv4, that means they can only hold 65,507 bytes because the 65,535 byte packet also includes header information) and delivery is not guaranteed as it is with TCP.

Echo Server

Since there is no connection, per se, the server does not need to listen for and accept connections. It only needs to use bind() to associate its socket with a port, and then wait for individual messages.

import socket
import sys

# Create a TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Bind the socket to the port
server_address = ('localhost', 10000)
print >>sys.stderr, 'starting up on %s port %s' % server_address

Messages are read from the socket using recvfrom(), which returns the data as well as the address of the client from which it was sent.

while True:
    print >>sys.stderr, '\nwaiting to receive message'
    data, address = sock.recvfrom(4096)
    print >>sys.stderr, 'received %s bytes from %s' % (len(data), address)
    print >>sys.stderr, data
    if data:
        sent = sock.sendto(data, address)
        print >>sys.stderr, 'sent %s bytes back to %s' % (sent, address)

Echo Client

The UDP echo client is similar the server, but does not use bind() to attach its socket to an address. It uses sendto() to deliver its message directly to the server, and recvfrom() to receive the response.

import socket
import sys

# Create a UDP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

server_address = ('localhost', 10000)
message = 'This is the message.  It will be repeated.'


    # Send data
    print >>sys.stderr, 'sending "%s"' % message
    sent = sock.sendto(message, server_address)

    # Receive response
    print >>sys.stderr, 'waiting to receive'
    data, server = sock.recvfrom(4096)
    print >>sys.stderr, 'received "%s"' % data

    print >>sys.stderr, 'closing socket'

Client and Server Together

Running the server produces:

$ python ./

starting up on localhost port 10000

waiting to receive message
received 42 bytes from ('', 50139)
This is the message.  It will be repeated.
sent 42 bytes back to ('', 50139)

waiting to receive message

and the client output is:

$ python ./

sending "This is the message.  It will be repeated."
waiting to receive
received "This is the message.  It will be repeated."
closing socket