=================================
      User Datagram Client and Server
     =================================
     
     The user datagram protocol (UDP) works differently from TCP/IP.  Where
     TCP is a *stream oriented* protocol, ensuring that all of the data is
     transmitted in the right order, UDP is a *message oriented* protocol.
     UDP does not require a long-lived connection, so setting up a UDP
     socket is a little simpler.  On the other hand, UDP messages must fit
     within a single packet (for IPv4, that means they can only hold 65,507
     bytes because the 65,535 byte packet also includes header information)
     and delivery is not guaranteed as it is with TCP.
     
     Echo Server
     ===========
     
     Since there is no connection, per se, the server does not need to
     listen for and accept connections.  It only needs to use :func:`bind`
     to associate its socket with a port, and then wait for individual
     messages.
     
     .. literalinclude:: socket_echo_server_dgram.py
        :lines: 10-19
     
     Messages are read from the socket using :func:`recvfrom`, which
     returns the data as well as the address of the client from which it
     was sent.
     
     .. literalinclude:: socket_echo_server_dgram.py
        :lines: 21-
     
     Echo Client
     ===========
     
     The UDP echo client is similar the server, but does not use
     :func:`bind` to attach its socket to an address.  It uses
     :func:`sendto` to deliver its message directly to the server, and
     :func:`recvfrom` to receive the response.
     
     .. include:: socket_echo_client_dgram.py
        :literal:
        :start-after: #end_pymotw_header
     
     Client and Server Together
     ==========================
     
     Running the server produces:
     
     ::
     
         $ python ./socket_echo_server_dgram.py 
         
         starting up on localhost port 10000
         
         waiting to receive message
         received 42 bytes from ('127.0.0.1', 50139)
         This is the message.  It will be repeated.
         sent 42 bytes back to ('127.0.0.1', 50139)
         
         waiting to receive message
         
     and the client output is:
     
     ::
     
         $ python ./socket_echo_client_dgram.py 
         
         sending "This is the message.  It will be repeated."
         waiting to receive
         received "This is the message.  It will be repeated."
         closing socket
         
         $