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

CHAPTER 5 ■ NETWORK DATA AND NETWORK ERRORS
which also supplies calls like socket() and bind(). I suggest that you ignore these awkward functions,
and use the struct module instead; it is more flexible, more general, and produces more readable code.
Framing and Quoting
If you are using UDP datagrams for communication, then the protocol itself takes the trouble to deliver
your data in discrete and identifiable chunks—and you have to reorder and re-transmit them yourself if
anything goes wrong on the network, as outlined in Chapter 2.
But if you have made the far more common option of using a TCP stream for communication, then
you will face the issue of framing—of how to delimit your messages so that the receiver can tell where
one message ends and the next begins. Since the data you supply to sendall() might be broken up into
several packets, the program that receives your message might have to make several recv() calls before
your whole message has been read.
The issue of framing asks the question: when is it safe for the receiver to finally stop calling recv()
and respond to your message?
As you might imagine, there are several approaches.
First, there is a pattern that can be used by extremely simple network protocols that involve only the
delivery of data—no response is expected, so there never has to come a time when the receiver decides
“Enough!” and turns around to send a response. In this case, the sender can loop until all of the outgoing
data has been passed to sendall() and then close() the socket. The receiver need only call recv()
repeatedly until the call finally returns an empty string, indicating that the sender has finally closed the
socket. You can see this pattern in Listing 5–1.
Listing 5–1. Sending a Single Stream of Data
#!/usr/bin/env python
# Foundations of Python Network Programming - Chapter 5 - streamer.py
# Client that sends data then closes the socket, not expecting a reply.
import socket, sys
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
HOST = sys.argv.pop() if len(sys.argv) == 3 else '127.0.0.1'
PORT = 1060
if sys.argv[1:] == ['server']:
» s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
» s.bind((HOST, PORT))
» s.listen(1)
» print 'Listening at', s.getsockname()
» sc, sockname = s.accept()
» print 'Accepted connection from', sockname
» sc.shutdown(socket.SHUT_WR)
» message = ''
» while True:
» » more = sc.recv(8192) # arbitrary value of 8k
» » if not more: # socket has closed when recv() returns ''
» » » break
» » message += more
» print 'Done receiving the message; it says:'
» print message
» sc.close()
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