doc/zmq_epgm.html

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<div id="header">
<h1>
zmq_pgm(7) Manual Page
</h1>
<h2>NAME</h2>
<div class="sectionbody">
<p>zmq_pgm -
   0MQ reliable multicast transport using PGM
</p>
</div>
</div>
<div id="content">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="paragraph"><p>PGM (Pragmatic General Multicast) is a protocol for reliable multicast
transport of data over IP networks.</p></div>
</div>
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>ØMQ implements two variants of PGM, the standard protocol where PGM datagrams
are layered directly on top of IP datagrams as defined by RFC 3208 (the <em>pgm</em>
transport) and "Encapsulated PGM" where PGM datagrams are encapsulated inside
UDP datagrams (the <em>epgm</em> transport).</p></div>
<div class="paragraph"><p>The <em>pgm</em> and <em>epgm</em> transports can only be used with the <em>ZMQ_PUB</em> and
<em>ZMQ_SUB</em> socket types.</p></div>
<div class="paragraph"><p>Further, PGM sockets are rate limited by default and incur a performance
penalty when used over a loop-back interface. For details, refer to the
<em>ZMQ_RATE</em>, <em>ZMQ_RECOVERY_IVL</em> and <em>ZMQ_MCAST_LOOP</em> options documented in
<a href="zmq_setsockopt.html">zmq_setsockopt(3)</a>.</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Caution</div>
</td>
<td class="content">The <em>pgm</em> transport implementation requires access to raw IP sockets.
Additional privileges may be required on some operating systems for this
operation. Applications not requiring direct interoperability with other PGM
implementations are encouraged to use the <em>epgm</em> transport instead which does
not require any special privileges.</td>
</tr></table>
</div>
</div>
<h2 id="_addressing">ADDRESSING</h2>
<div class="sectionbody">
<div class="paragraph"><p>A ØMQ address string consists of two parts as follows:
<em>transport</em><tt>://</tt><em>endpoint</em>. The <em>transport</em> part specifies the underlying
transport protocol to use. For the standard PGM protocol, <em>transport</em> shall be
set to <tt>pgm</tt>. For the "Encapsulated PGM" protocol <em>transport</em> shall be set to
<tt>epgm</tt>. The meaning of the <em>endpoint</em> part for both the <em>pgm</em> and <em>epgm</em>
transport is defined below.</p></div>
<h3 id="_connecting_a_socket">Connecting a socket</h3><div style="clear:left"></div>
<div class="paragraph"><p>When connecting a socket to a peer address using <em>zmq_connect()</em> with the <em>pgm</em>
or <em>epgm</em> transport, the <em>endpoint</em> shall be interpreted as an <em>interface</em>
followed by a semicolon, followed by a <em>multicast address</em>, followed by a colon
and a port number.</p></div>
<div class="paragraph"><p>An <em>interface</em> may be specified by either of the following:</p></div>
<div class="ulist"><ul>
<li>
<p>
The interface name as defined by the operating system.
</p>
</li>
<li>
<p>
The primary IPv4 address assigned to the interface, in it&#8217;s numeric
  representation.
</p>
</li>
</ul></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">Interface names are not standardised in any way and should be assumed to
be arbitrary and platform dependent. On Win32 platforms no short interface
names exist, thus only the primary IPv4 address may be used to specify an
<em>interface</em>.</td>
</tr></table>
</div>
<div class="paragraph"><p>A <em>multicast address</em> is specified by an IPv4 multicast address in it&#8217;s numeric
representation.</p></div>
</div>
<h2 id="_wire_format">WIRE FORMAT</h2>
<div class="sectionbody">
<div class="paragraph"><p>Consecutive PGM datagrams are interpreted by ØMQ as a single continuous stream
of data where ØMQ messages are not necessarily aligned with PGM datagram
boundaries and a single ØMQ message may span several PGM datagrams. This stream
of data consists of ØMQ messages encapsulated in <em>frames</em> as described in
<a href="zmq_tcp.html">zmq_tcp(7)</a>.</p></div>
<h3 id="_pgm_datagram_payload">PGM datagram payload</h3><div style="clear:left"></div>
<div class="paragraph"><p>The following ABNF grammar represents the payload of a single PGM datagram as
used by ØMQ:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>datagram               = (offset data)
offset                 = 2OCTET
data                   = *OCTET</tt></pre>
</div></div>
<div class="paragraph"><p>In order for late joining consumers to be able to identify message boundaries,
each PGM datagram payload starts with a 16-bit unsigned integer in network byte
order specifying either the offset of the first message <em>frame</em> in the datagram
or containing the value <tt>0xFFFF</tt> if the datagram contains solely an
intermediate part of a larger message.</p></div>
<div class="paragraph"><p>Note that offset specifies where the first message begins rather than the first
message part. Thus, if there are trailing message parts at the beginning of
the packet the offset ignores them and points to first initial message part
in the packet.</p></div>
<div class="paragraph"><p>The following diagram illustrates the layout of a single PGM datagram payload:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>+------------------+----------------------+
| offset (16 bits) |         data         |
+------------------+----------------------+</tt></pre>
</div></div>
<div class="paragraph"><p>The following diagram further illustrates how three example ØMQ frames are laid
out in consecutive PGM datagram payloads:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>First datagram payload
+--------------+-------------+---------------------+
| Frame offset |   Frame 1   |   Frame 2, part 1   |
|    0x0000    | (Message 1) | (Message 2, part 1) |
+--------------+-------------+---------------------+

Second datagram payload
+--------------+---------------------+
| Frame offset |   Frame 2, part 2   |
| 0xFFFF       | (Message 2, part 2) |
+--------------+---------------------+

Third datagram payload
+--------------+----------------------------+-------------+
| Frame offset |   Frame 2, final 8 bytes   |   Frame 3   |
| 0x0008       | (Message 2, final 8 bytes) | (Message 3) |
+--------------+----------------------------+-------------+</tt></pre>
</div></div>
</div>
<h2 id="_example">EXAMPLE</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="title">Connecting a socket</div>
<div class="content">
<pre><tt>/* Connecting to the multicast address 239.192.1.1, port 5555, */
/* using the first Ethernet network interface on Linux */
/* and the Encapsulated PGM protocol */
rc = zmq_connect(socket, "epgm://eth0;239.192.1.1:5555");
assert (rc == 0);
/* Connecting to the multicast address 239.192.1.1, port 5555, */
/* using the network interface with the address 192.168.1.1 */
/* and the standard PGM protocol */
rc = zmq_connect(socket, "pgm://192.168.1.1;239.192.1.1:5555");
assert (rc == 0);</tt></pre>
</div></div>
</div>
<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph"><p><a href="zmq_connect.html">zmq_connect(3)</a>
<a href="zmq_setsockopt.html">zmq_setsockopt(3)</a>
<a href="zmq_tcp.html">zmq_tcp(7)</a>
<a href="zmq_ipc.html">zmq_ipc(7)</a>
<a href="zmq_inproc.html">zmq_inproc(7)</a>
<a href="zmq.html">zmq(7)</a></p></div>
</div>
<h2 id="_authors">AUTHORS</h2>
<div class="sectionbody">
<div class="paragraph"><p>This ØMQ manual page was written by Martin Sustrik &lt;<a href="mailto:sustrik@250bpm.com">sustrik@250bpm.com</a>&gt; and
Martin Lucina &lt;<a href="mailto:mato@kotelna.sk">mato@kotelna.sk</a>&gt;.</p></div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
ØMQ 2.1.5<br />
Last updated 2011-04-16 10:29:20 CEST
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