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<li><a class="reference internal" href="#">User Datagram Client and Server</a><ul>
<li><a class="reference internal" href="#echo-server">Echo Server</a></li>
<li><a class="reference internal" href="#echo-client">Echo Client</a></li>
<li><a class="reference internal" href="#client-and-server-together">Client and Server Together</a></li>
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<div class="section" id="user-datagram-client-and-server">
<h1>User Datagram Client and Server<a class="headerlink" href="#user-datagram-client-and-server" title="Permalink to this headline">¶</a></h1>
<p>The user datagram protocol (UDP) works differently from TCP/IP. Where
TCP is a <em>stream oriented</em> protocol, ensuring that all of the data is
transmitted in the right order, UDP is a <em>message oriented</em> 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.</p>
<div class="section" id="echo-server">
<h2>Echo Server<a class="headerlink" href="#echo-server" title="Permalink to this headline">¶</a></h2>
<p>Since there is no connection, per se, the server does not need to
listen for and accept connections. It only needs to use <tt class="xref py py-func docutils literal"><span class="pre">bind()</span></tt>
to associate its socket with a port, and then wait for individual
messages.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_DGRAM</span><span class="p">)</span>
<span class="c"># Bind the socket to the port</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'starting up on </span><span class="si">%s</span><span class="s"> port </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">server_address</span>
<span class="n">sock</span><span class="o">.</span><span class="n">bind</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
</pre></div>
</div>
<p>Messages are read from the socket using <tt class="xref py py-func docutils literal"><span class="pre">recvfrom()</span></tt>, which
returns the data as well as the address of the client from which it
was sent.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'</span><span class="se">\n</span><span class="s">waiting to receive message'</span>
<span class="n">data</span><span class="p">,</span> <span class="n">address</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">recvfrom</span><span class="p">(</span><span class="mi">4096</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received </span><span class="si">%s</span><span class="s"> bytes from </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">),</span> <span class="n">address</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="n">data</span>
<span class="k">if</span> <span class="n">data</span><span class="p">:</span>
<span class="n">sent</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">sendto</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">address</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'sent </span><span class="si">%s</span><span class="s"> bytes back to </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">sent</span><span class="p">,</span> <span class="n">address</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="echo-client">
<h2>Echo Client<a class="headerlink" href="#echo-client" title="Permalink to this headline">¶</a></h2>
<p>The UDP echo client is similar the server, but does not use
<tt class="xref py py-func docutils literal"><span class="pre">bind()</span></tt> to attach its socket to an address. It uses
<tt class="xref py py-func docutils literal"><span class="pre">sendto()</span></tt> to deliver its message directly to the server, and
<tt class="xref py py-func docutils literal"><span class="pre">recvfrom()</span></tt> to receive the response.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a UDP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_DGRAM</span><span class="p">)</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="n">message</span> <span class="o">=</span> <span class="s">'This is the message. It will be repeated.'</span>
<span class="k">try</span><span class="p">:</span>
<span class="c"># Send data</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'sending "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">message</span>
<span class="n">sent</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">sendto</span><span class="p">(</span><span class="n">message</span><span class="p">,</span> <span class="n">server_address</span><span class="p">)</span>
<span class="c"># Receive response</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'waiting to receive'</span>
<span class="n">data</span><span class="p">,</span> <span class="n">server</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">recvfrom</span><span class="p">(</span><span class="mi">4096</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">data</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'closing socket'</span>
<span class="n">sock</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="client-and-server-together">
<h2>Client and Server Together<a class="headerlink" href="#client-and-server-together" title="Permalink to this headline">¶</a></h2>
<p>Running the server produces:</p>
<div class="highlight-python"><pre>$ 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</pre>
</div>
<p>and the client output is:</p>
<div class="highlight-python"><pre>$ 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
$</pre>
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