UDP Networking

Overview

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.

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 twisted.internet.interfaces.IReactorUDP reactor API.

DatagramProtocol

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 twisted.internet.protocol.DatagramProtocol. 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.

Here is a simple example:

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()

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 twisted.internet.protocol.DatagramProtocol.startProtocol and twisted.internet.protocol.DatagramProtocol.stopProtocol methods that will get called when the protocol is connected and disconnected from a UDP socket.

The protocol's transport attribute will implement the twisted.internet.interfaces.IUDPTransport interface. Notice that the host argument should be an IP, not a hostname. If you only have the hostname use reactor.resolve() to resolve the address (see twisted.internet.interfaces.IReactorCore.resolve).

Connected UDP

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 it may provide notification of undelivered packages. This depends on many factors, almost all of which are out of the control of the application, but it still presents certain benefits which occassionally make it useful.

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.

from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor

class Helloer(DatagramProtocol):

    def startProtocol(self):
        self.transport.connect("192.168.1.1", 1234)
        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)

    # Possibly invoked if there is no server listening on the
    # address to which we are sending.
    def connectionRefused(self):
        print "No one listening"

# 0 means any port, we don't care in this case
reactor.listenUDP(0, Helloer())
reactor.run()

Note that connect(), like write() will only accept IP addresses, not unresolved domain names. To obtain the IP of a domain name use reactor.resolve(), e.g.:

from twisted.internet import reactor

def gotIP(ip):
    print "IP of 'example.com' is", ip

reactor.resolve('example.com').addCallback(gotIP)

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.

Multicast UDP

A multicast UDP socket can send and receive datagrams from multiple clients. The interesting and useful feature of the multicast is that a client can contact multiple servers with a single packet, without knowing the specific IP of any of the hosts.

from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
from twisted.application.internet import MulticastServer

class MulticastServerUDP(DatagramProtocol):
    def startProtocol(self):
        print 'Started Listening'
        # Join a specific multicast group, which is the IP we will respond to
        self.transport.joinGroup('224.0.0.1')

    def datagramReceived(self, datagram, address):
        # The uniqueID check is to ensure we only service requests from
        # ourselves
        if datagram == 'UniqueID':
            print "Server Received:" + repr(datagram)
            self.transport.write("data", address)

# Note that the join function is picky about having a unique object
# on which to call join.  To avoid using startProtocol, the following is
# sufficient:
#reactor.listenMulticast(8005, MulticastServerUDP()).join('224.0.0.1')

# Listen for multicast on 224.0.0.1:8005
reactor.listenMulticast(8005, MulticastServerUDP())
reactor.run()

The server protocol is very simple, and closely resembles a normal listenUDP implementation. The main difference is that instead of listenUDP, listenMulticast is called with a specified port number. The server must also call joinGroup to specify on which multicast IP address it will service requests. Another item of interest is the contents of the datagram. Many different applications use multicast as a way of device discovery, which leads to an abundance of packets flying around. Checking the payload can ensure that we only service requests from our specific clients.

from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
from twisted.application.internet import MulticastServer

class MulticastClientUDP(DatagramProtocol):

    def datagramReceived(self, datagram, address):
            print "Received:" + repr(datagram)

# Send multicast on 224.0.0.1:8005, on our dynamically allocated port
reactor.listenUDP(0, MulticastClientUDP()).write('UniqueID',
                                                 ('224.0.0.1', 8005))
reactor.run()

This is a mirror implementation of a standard UDP client. The only difference is that the destination IP is the multicast address. This datagram will be distributed to every server listening on 224.0.0.1 and port 8005. Note that the client port is specified as 0, as we have no need to keep track of what port the client is listening on.

Acknowledgements

Thank you to all contributors to this document, including:

• Kyle Robertson, author of the explanation and examples of multicast