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<div id="header">
<h1>nng_pair(7) Manual Page</h1>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_synopsis">SYNOPSIS</a></li>
<li><a href="#_description">DESCRIPTION</a>
<ul class="sectlevel2">
<li><a href="#_socket_operations">Socket Operations</a></li>
<li><a href="#_protocol_versions">Protocol Versions</a></li>
<li><a href="#_polyamorous_mode">Polyamorous Mode</a></li>
<li><a href="#_protocol_options">Protocol Options</a></li>
<li><a href="#_protocol_headers">Protocol Headers</a></li>
</ul>
</li>
<li><a href="#_see_also">SEE ALSO</a></li>
</ul>
</div>
<h2>NAME</h2>
<div class="sectionbody">
<p>nng_pair - pair protocol</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="title">Version 0</div>
<div class="content">
<pre class="pygments highlight"><code data-lang="c"><span></span><span class="tok-cp">#include</span> <span class="tok-cpf"><nng/protocol/pair0/pair.h></span><span class="tok-cp"></span>
<span class="tok-kt">int</span> <span class="tok-nf">nng_pair0_open</span><span class="tok-p">(</span><span class="tok-n">nng_socket</span> <span class="tok-o">*</span><span class="tok-n">s</span><span class="tok-p">);</span></code></pre>
</div>
</div>
<div class="listingblock">
<div class="title">Version 1</div>
<div class="content">
<pre class="pygments highlight"><code data-lang="c"><span></span><span class="tok-cp">#include</span> <span class="tok-cpf"><nng/protocol/pair1/pair.h></span><span class="tok-cp"></span>
<span class="tok-kt">int</span> <span class="tok-nf">nng_pair1_open</span><span class="tok-p">(</span><span class="tok-n">nng_socket</span> <span class="tok-o">*</span><span class="tok-n">s</span><span class="tok-p">);</span></code></pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The <em>nng_pair</em> protocol implements a peer-to-peer pattern, where
relationships between peers are one-to-one.</p>
</div>
<div class="paragraph">
<p>Version 1 of this protocol supports an optional <em>polyamorous</em> mode where a
peer can maintain multiple partnerships. Using this mode requires
some additional sophistication in the application.</p>
</div>
<div class="sect2">
<h3 id="_socket_operations">Socket Operations</h3>
<div class="paragraph">
<p>The <code>nng_pair_open()</code> call creates a <em>pair</em> socket. Normally, this
pattern will block when attempting to send a message, if no peer is
able to receive the message.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
Even though this mode may appear to be "reliable", because back-pressure
prevents discarding messages most of the time, there are topologies involving
<em>devices</em> (see <a href="nng_device.html">nng_device(3)</a>) or raw mode sockets where
messages may be discarded. Applications that require reliable delivery
semantics should consider using <a href="nng_req.html">nng_req(7)</a> sockets, or
implement their own acknowledgement layer on top of pair sockets.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>In order to avoid head-of-line blocking conditions, <em>polyamorous</em> mode pair
sockets (version 1 only) discard messages if they are unable to deliver them
to a peer.</p>
</div>
</div>
<div class="sect2">
<h3 id="_protocol_versions">Protocol Versions</h3>
<div class="paragraph">
<p>Version 0 is the legacy version of this protocol. It lacks any header
information, and is suitable when building simple one-to-one topologies.</p>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
Use version 0 if you need to communicate with other implementations,
including the legacy <a href="https://github.com/nanomsg/nanomsg">nanomsg</a> library or
<a href="https://github.com/go-mangos/mangos">mangos</a>.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Version 1 of the protocol offers improved protection against loops when
used with <a href="nng_device.html">nng_device(3)</a>. It also offers <em>polyamorous</em>
mode for forming multiple partnerships on a single socket.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
Version 1 of this protocol is considered experimental at this time.
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_polyamorous_mode">Polyamorous Mode</h3>
<div class="paragraph">
<p>Normally pair sockets are for one-to-one communication, and a given peer
will reject new connections if it already has an active connection to another
peer.</p>
</div>
<div class="paragraph">
<p>In <em>polyamorous</em> mode, which is only available with version 1, a socket can
support many one-to-one connections. In this mode, the application must
choose the remote peer to receive an ougoing message by setting the value
of the pipe ID on the outgoing message using
the <a href="nng_msg_set_pipe.html">nng_msg_set_pipe(3)</a> function.</p>
</div>
<div class="paragraph">
<p>Most often the value of the outgoing pipe ID will be obtained from an incoming
message using the <a href="nng_msg_get_pipe.html">nng_msg_get_pipe(3)</a> function,
such as when replying to an incoming message.</p>
</div>
<div class="paragraph">
<p>In order to prevent head-of-line blocking, if the peer on the given pipe
is not able to receive (or the pipe is no longer available, such as if the
peer has disconnected), then the message will be discarded with no notification
to the sender.</p>
</div>
</div>
<div class="sect2">
<h3 id="_protocol_options">Protocol Options</h3>
<div class="paragraph">
<p>The following protocol-specific options are available.</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>NNG_OPT_PAIR1_POLY</code></dt>
<dd>
<p>(Version 1 only). This option enables the use of <em>polyamorous</em> mode.
The value is read-write, and takes an integer boolean value. The default
false value (0) indicates that legacy monogamous mode should be used.</p>
</dd>
<dt class="hdlist1"><code>NNG_OPT_MAXTTL</code></dt>
<dd>
<p>(Version 1 only). Maximum time-to-live. This option is an integer value
between 0 and 255,
inclusive, and is the maximum number of "hops" that a message may
pass through until it is discarded. The default value is 8. A value
of 0 may be used to disable the loop protection, allowing an infinite
number of hops.</p>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
Each node along a forwarding path may have it’s own value for the
maximum time-to-live, and performs its own checks before forwarding a message.
Therefore it is helpful if all nodes in the topology use the same value for
this option.
</td>
</tr>
</table>
</div>
</dd>
</dl>
</div>
</div>
<div class="sect2">
<h3 id="_protocol_headers">Protocol Headers</h3>
<div class="paragraph">
<p>Version 0 of the pair protocol has no protocol-specific headers.</p>
</div>
<div class="paragraph">
<p>Version 1 of the pair protocol uses a single 32-bit unsigned value. The
low-order (big-endian) byte of this value contains a "hop" count, and is
used in conjuction with the <code>NNG_OPT_MAXTTL</code> option to guard against
device forwarding loops. This value is initialized to 1, and incremented
each time the message is received by a new node.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="nng.html">nng(7)</a></p>
</div>
</div>
</div>
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