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<title>UDP Networking</title>
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<h1>UDP Networking</h1>
<h2>Overview</h2>
<p>Unlike TCP, UDP has no notion of connections. A UDP socket can receive
datagrams from any server on the network, and send datagrams to any host
on the network. In addition, datagrams may arrive in any order, never
arrive at all, or be duplicated in transit.</p>
<p>Since there are no multiple connections, we only use a single object,
a protocol, for each UDP socket. We then use the reactor to connect
this protocol to a UDP transport, using the
<code class="API">twisted.internet.interfaces.IReactorUDP</code>
reactor API.</p>
<h2>DatagramProtocol</h2>
<p>At the base, the place where you actually implement the protocol parsing
and handling, is the DatagramProtocol class. This class will usually be decended
from <code class="API">twisted.internet.protocol.DatagramProtocol</code>. Most
protocol handlers inherit either from this class or from one of its
convenience children. The DatagramProtocol class receives datagrams, and can
send them out over the network. Received datagrams include the address they
were sent from, and when sending datagrams the address to send to must
be specified.</p>
<p>Here is a simple example:</p>
<pre class="python">
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class Echo(DatagramProtocol):
def datagramReceived(self, data, (host, port)):
print "received %r from %s:%d" % (data, host, port)
self.transport.write(data, (host, port))
reactor.listenUDP(9999, Echo())
reactor.run()
</pre>
<p>As you can see, the protocol is registed with the reactor.
This means it may be persisted if it's added to an application,
and thus it has
<code class="API">twisted.internet.protocol.DatagramProtocol.startProtocol</code> and
<code class="API">twisted.internet.protocol.DatagramProtocol.stopProtocol</code> methods that
will get called when the protocol is connected and disconnected
from a UDP socket.</p>
<p>The protocol's <code class="python">transport</code> attribute will implement the
<code class="API">twisted.internet.interfaces.IUDPTransport</code>
interface. Notice that the <code class="python">host</code> argument should be an IP,
not a hostname. If you only have the hostname use <code class="python">reactor.resolve()</code>
to resolve the address (see <code class="API">twisted.internet.interfaces.IReactorCore.resolve</code>).</p>
<h2>Connected UDP</h2>
<p>A connected UDP socket is slighly different from a standard one - it can
only send and receive datagrams to/from a single address, but this does
not in any way imply a connection. Datagrams may still arrive in any order,
and the port on the other side may have no one listening. The benefit
of the connected UDP socket is that it is faster.</p>
<p>Unlike a regular UDP protocol, we do not need to specify where to
send datagrams to, and are not told where they came from since
they can only come from address the socket is 'connected' to.</p>
<pre class="python">
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class Helloer(DatagramProtocol):
def startProtocol(self):
d = self.transport.connect("192.168.1.1", 1234)
d.addCallback(self._cbConnected)
def _cbConnected(self, (host, port)):
print "we can only send to %s now" % str((host, port))
self.transport.write("hello") # no need for address
def datagramReceived(self, data, (host, port)):
print "received %r from %s:%d" % (data, host, port)
# 0 means any port, we don't care in this case
reactor.listenUDP(0, Helloer())
reactor.run()
</pre>
<p>Note that <code class="python">connect()</code>, like <code
class="python">write()</code> will only accept IP addresses, not
unresolved domain names. To obtain the IP of a domain name use <code
class="python">reactor.resolve()</code>, e.g.:</p>
<pre class="python">
from twisted.internet import reactor
def gotIP(ip):
print "IP of 'example.com' is", ip
reactor.resolve('example.com').addCallback(gotIP)
</pre>
<p>Connecting to a new address after a previous connection, or
making a connected port unconnected are not currently supported,
but will likely be supported in the future.</p>
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