The foregoing program listings have suggested that a UDP packet can be up to 64kB in size, whereas you probably already know that your Ethernet or wireless card can only handle packets of around 1,500 bytes instead.
The actual truth is that IP sends small UDP packets as single packets on the wire, but splits up larger UDP packets into several small physical packets. This means that large packets are more likely to be dropped, since if any one of their pieces fails to make its way to the destination, then the whole packet can never be reassembled and delivered to the listening operating system. But aside from the higher chance of failure, this process of fragmenting large UDP packets so that they will fit on the wire should be invisible to your application. There are three ways, however, in which it might be relevant:
Linux is one operating system that supports this last option. Take a look at
big_sender.py , which sends
a very large message to one of the servers that we have just designed.
import IN, socket, sys s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) MAX = 65535 PORT = 1060 if len(sys.argv) != 2: print >>sys.stderr, 'usage: big_sender.py host' sys.exit(2) hostname = sys.argv s.connect((hostname, PORT)) s.setsockopt(socket.IPPROTO_IP, IN.IP_MTU_DISCOVER, IN.IP_PMTUDISC_DO) try: s.send('#' * 65000) except socket.error: print 'The message did not make it' option = getattr(IN, 'IP_MTU', 14) # constant taken from <linux/in.h> print 'MTU:', s.getsockopt(socket.IPPROTO_IP, option) else: print 'The big message was sent! Your network supports really big packets!'
If we run this program against a server elsewhere on my home network, then we discover that my wireless network allows physical packets that are no bigger than the 1,500 bytes typically supported by Ethernet-style networks:
root@erlerobot:~/Python_files# python big_sender.py 127.0.0.0 The message did not make it MTU: 1500