Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 2 ■ UDP
Unreliability, Backoff, Blocking, Timeouts
Because the client and server in the previous section were both running on the same machine and
talking through its loopback interface—which is not even a physical network card that could experience
a signaling glitch and lose a packet, but merely a virtual connection back to the same machine deep in
the network stack—there was no real way that packets could get lost, and so we did not actually see any
of the inconvenience of UDP. How does code change when packets could really be lost?
Take a look at Listing 2–2. Unlike the previous example, you can run this client and server on two
different machines on the Internet. And instead of always answering client requests, this server
randomly chooses to answer only half of the requests coming in from clients—which will let us
demonstrate how to build reliability into our client code, without waiting what might be hours for a real
dropped packet to occur!
Listing 2–2. UDP Server and Client on Different Machines
#!/usr/bin/env python
# Foundations of Python Network Programming - Chapter 2 - udp_remote.py
# UDP client and server for talking over the network
import random, socket, sys
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
MAX = 65535
PORT = 1060
if 2 2.0:
22