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<li><a class="reference internal" href="#">Multicast</a><ul>
<li><a class="reference internal" href="#sending-multicast-messages">Sending Multicast Messages</a></li>
<li><a class="reference internal" href="#receiving-multicast-messages">Receiving Multicast Messages</a></li>
<li><a class="reference internal" href="#example-output">Example Output</a></li>
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<div class="section" id="multicast">
<h1>Multicast<a class="headerlink" href="#multicast" title="Permalink to this headline">¶</a></h1>
<p>Point-to-point connections handle a lot of communication needs, but
passing the same information between many peers becomes challenging as
the number of direct connections grows. Sending messages separately
to each recipient consumes additional processing time and bandwidth,
which can be a problem for applications such as streaming video or
audio. Using <em>multicast</em> to deliver messages to more than one
endpoint at a time achieves better efficiency because the network
infrastructure ensures that the packets are delivered to all
recipients.</p>
<p>Multicast messages are always sent using UDP, since TCP requires an
end-to-end communication channel. The addresses for multicast, called
<em>multicast groups</em>, are a subset of regular IPv4 address range
(224.0.0.0 through 230.255.255.255) reserved for multicast traffic.
These addresses are treated specially by network routers and switches,
so messages sent to the group can be distributed over the Internet to
all recipients that have joined the group.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Some managed switches and routers have multicast traffic disabled
by default. If you have trouble with the example programs, check
your network hardware settings.</p>
</div>
<div class="section" id="sending-multicast-messages">
<h2>Sending Multicast Messages<a class="headerlink" href="#sending-multicast-messages" title="Permalink to this headline">¶</a></h2>
<p>This modified echo client will send a message to a multicast group,
then report all of the responses it receives. Since it has no way of
knowing how many responses to expect, it uses a timeout value on the
socket to avoid blocking indefinitely waiting for an answer.</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">struct</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="n">message</span> <span class="o">=</span> <span class="s">'very important data'</span>
<span class="n">multicast_group</span> <span class="o">=</span> <span class="p">(</span><span class="s">'224.3.29.71'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="c"># Create the datagram 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"># Set a timeout so the socket does not block indefinitely when trying</span>
<span class="c"># to receive data.</span>
<span class="n">sock</span><span class="o">.</span><span class="n">settimeout</span><span class="p">(</span><span class="mf">0.2</span><span class="p">)</span>
</pre></div>
</div>
<p>The socket also needs to be configured with a <em>time-to-live</em> value
(TTL) for the messages. The TTL controls how many networks will
receive the packet. Set the TTL with the <tt class="xref py py-const docutils literal"><span class="pre">IP_MULTICAST_TTL</span></tt>
option and <tt class="xref py py-func docutils literal"><span class="pre">setsockopt()</span></tt>. The default, <tt class="docutils literal"><span class="pre">1</span></tt>, means that the
packets are not forwarded by the router beyond the current network
segment. The value can range up to 255, and should be packed into a
single byte.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Set the time-to-live for messages to 1 so they do not go past the</span>
<span class="c"># local network segment.</span>
<span class="n">ttl</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s">'b'</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="n">sock</span><span class="o">.</span><span class="n">setsockopt</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">IPPROTO_IP</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">IP_MULTICAST_TTL</span><span class="p">,</span> <span class="n">ttl</span><span class="p">)</span>
</pre></div>
</div>
<p>The rest of the sender looks like the UDP echo client, except that it
expects multiple responses so uses a loop to call <tt class="xref py py-func docutils literal"><span class="pre">recvfrom()</span></tt>
until it times out.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="c"># Send data to the multicast group</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">multicast_group</span><span class="p">)</span>
<span class="c"># Look for responses from all recipients</span>
<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">'waiting to receive'</span>
<span class="k">try</span><span class="p">:</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">16</span><span class="p">)</span>
<span class="k">except</span> <span class="n">socket</span><span class="o">.</span><span class="n">timeout</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">'timed out, no more responses'</span>
<span class="k">break</span>
<span class="k">else</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">" from </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">server</span><span class="p">)</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="receiving-multicast-messages">
<h2>Receiving Multicast Messages<a class="headerlink" href="#receiving-multicast-messages" title="Permalink to this headline">¶</a></h2>
<p>The first step to establishing a multicast receiver is to create the
UDP socket.</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">struct</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="n">multicast_group</span> <span class="o">=</span> <span class="s">'224.3.29.71'</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">''</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="c"># Create the 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 to the 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>After the regular socket is created and bound to a port, it can be
added to the multicast group by using <tt class="xref py py-func docutils literal"><span class="pre">setsockopt()</span></tt> to change the
<tt class="xref py py-const docutils literal"><span class="pre">IP_ADD_MEMBERSHIP</span></tt> option. The option value is the 8-byte
packed representation of the multicast group address followed by the
network interface on which the server should listen for the traffic,
identified by its IP address. In this case, the receiver listens on
all interfaces using <tt class="xref py py-const docutils literal"><span class="pre">INADDR_ANY</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Tell the operating system to add the socket to the multicast group</span>
<span class="c"># on all interfaces.</span>
<span class="n">group</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">inet_aton</span><span class="p">(</span><span class="n">multicast_group</span><span class="p">)</span>
<span class="n">mreq</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s">'4sL'</span><span class="p">,</span> <span class="n">group</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">INADDR_ANY</span><span class="p">)</span>
<span class="n">sock</span><span class="o">.</span><span class="n">setsockopt</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">IPPROTO_IP</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">IP_ADD_MEMBERSHIP</span><span class="p">,</span> <span class="n">mreq</span><span class="p">)</span>
</pre></div>
</div>
<p>The main loop for the receiver is just like the regular UDP echo
server.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Receive/respond loop</span>
<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">1024</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">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 acknowledgement to'</span><span class="p">,</span> <span class="n">address</span>
<span class="n">sock</span><span class="o">.</span><span class="n">sendto</span><span class="p">(</span><span class="s">'ack'</span><span class="p">,</span> <span class="n">address</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="example-output">
<h2>Example Output<a class="headerlink" href="#example-output" title="Permalink to this headline">¶</a></h2>
<p>This example shows the multicast receiver running on two different
hosts, <tt class="docutils literal"><span class="pre">A</span></tt> has address <tt class="docutils literal"><span class="pre">192.168.1.17</span></tt> and <tt class="docutils literal"><span class="pre">B</span></tt> has address
<tt class="docutils literal"><span class="pre">192.168.1.8</span></tt>.</p>
<div class="highlight-python"><pre>[A]$ python ./socket_multicast_receiver.py
waiting to receive message
received 19 bytes from ('192.168.1.17', 51382)
very important data
sending acknowledgement to ('192.168.1.17', 51382)
[B]$ python ./socket_multicast_receiver.py
binding to ('', 10000)
waiting to receive message
received 19 bytes from ('192.168.1.17', 51382)
very important data
sending acknowledgement to ('192.168.1.17', 51382)</pre>
</div>
<p>The sender is running on host <tt class="docutils literal"><span class="pre">A</span></tt>.</p>
<div class="highlight-python"><pre>$ python ./socket_multicast_sender.py
sending "very important data"
waiting to receive
received "ack" from ('192.168.1.17', 10000)
waiting to receive
received "ack" from ('192.168.1.8', 10000)
waiting to receive
timed out, no more responses
closing socket</pre>
</div>
<p>The message is sent one time, and two acknowledgements of the outgoing
message are received, one from each of host <tt class="docutils literal"><span class="pre">A</span></tt> and <tt class="docutils literal"><span class="pre">B</span></tt>.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference external" href="http://en.wikipedia.org/wiki/Multicast">Wikipedia: Multicast</a></dt>
<dd>Article describing technical details of multicasting.</dd>
<dt><a class="reference external" href="http://en.wikipedia.org/wiki/IP_multicast">Wikipedia: IP Multicast</a></dt>
<dd>Article about IP multicasting, with information about addressing.</dd>
</dl>
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