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

CHAPTER 2 ■ UDP
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
» 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:
$ python big_sender.py guinness
The message did not make it
MTU: 1500
It is slightly more surprising that the loopback interface on my laptop, which presumably could
support packets as large as my RAM, also imposes an MTU that is far short of the maximum UDP packet
length:
$ python big_sender.py localhost
The message did not make it
MTU: 16436
But, the ability to check the MTU is a fairly obscure feature. As you can see from the program listing,
Python 2.6.5 on my machine for some reason fails to include the IP_MTU socket option name that is
necessary to determine a socket’s current MTU, so I had to manually copy the integer option code 14 out
of one of the system C header files. So you should probably ignore the issue of fragmentation and, if you
worry about it at all, try to keep your UDP packets short; but this example was at least useful in showing
you that fragmentation does need to take place, in case you run into any of its consequences!
Socket Options
The POSIX socket interface also supports all sorts of socket options that control specific behaviors of
network sockets. These are accessed through the Python socket methods getsockopt() and setsockopt(),
using the options you will find documented for your operating system. On Linux, for example, try viewing
the manual pages socket(7), udp(7), and—when you progress to the next chapter—tcp(7).
When setting socket options, you have to first name the “option group” in which they live, and then
as a subsequent argument name the actual option you want to set; consult your operating system
manual for the names of these groups. See Listing 2–3 in the next section for some example real-world
calls involving socket options. Just like the Python calls getattr() and setattr(), the set call simply
takes one more argument:
value = s.getsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST)
s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, value)
Many options are specific to particular operating systems, and may be finicky about how their
options are presented. Here are some of the more common:
• SO_BROADCAST: Allows broadcast UDP packets to be sent and received; see the next
section for details.
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