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</head>
<body class="article">
<div id="header">
<h1>Pair Scalability Protocol v1</h1>
<div class="details">
<span id="author" class="author">Garrett D'Amore</span><br>
<span id="email" class="email"><a href="mailto:garrett@damore.org">garrett@damore.org</a></span><br>
<span id="revnumber">version 1.0,</span>
<span id="revdate">2017-11-08</span>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p><span class="image"><img src="https://img.shields.io/badge/status-draft-yellow.svg" alt="draft"></span></p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_pair_protocol">Pair Protocol</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Status: draft</p>
</li>
<li>
<p>Authors: <a href="mailto:garrett@damore.org">Garrett D’Amore</a></p>
</li>
<li>
<p>Version: 1.0</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_abstract">Abstract</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This document defines a scalability protocol used for bidirectional
stateless communications between a pair of nodes.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_license">License</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Copyright 2017 <a href="mailto:garrett@damore.org">Garrett D’Amore</a></p>
</div>
<div class="paragraph">
<p>This specification is licensed under the Apache License, Version 2.0
(the "License"); you may not use this file except in compliance with the
License.
You may obtain a copy of the license
<a href="http://www.apache.org/licenses/LICENSE-2.0">online</a>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_language">Language</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in <a href="https://tools.ietf.org/html/rfc2119">RFC 2119</a>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_goals">Goals</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This document describes a <em>PAIR</em> protocol, which is to be used between
a pair of nodes for message oriented but otherwise free form bidirectional
communication between two parties.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
An earlier, undocumented, version of this protocol was used in
<a href="http://www.nanomsg.org">nanomsg</a> version 1 and compatible implementations.
<sup class="footnote">[<a id="_footnoteref_1" class="footnote" href="#_footnote_1" title="View footnote.">1</a>]</sup>
While new implementations are encouraged to use this version, they
<strong>MAY</strong> elect to use the legacy protocol at the discretion of the user.
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_usage_scenarios">Usage Scenarios</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The use case for this protocol is one where a pair of potential
peers wishes to communicate bidirectionally with little or no constraint on
message contents, or replies. As such this protocol itself is
unacknowledged. These are typical use cases:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>One client sends and receives messages with one server.
This is the historical model.</p>
</li>
<li>
<p>Many clients send and receive messages with one server.
The server sends and receives messages with clients, but must identify
which client to which it desires to send a message.</p>
</li>
<li>
<p>One client sends and receives messages with many servers. The
client must determine to which server it desires to send any
particular message.</p>
</li>
<li>
<p>Combining the above, many clients and many servers communicate,
forming a kind of mesh, but every sender identifies the intended
recipient explicitly.</p>
</li>
</ol>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_head_of_line_blocking">Head of Line Blocking</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Note that in the <a href="#_usage_scenarios">usage scenarios</a> involving
polyamrous relationships, since
a single socket is being used to communicate to potentially many peers,
we cannot use a blocking paradigm; that is we must not allow a slow peer
to block communications with other peers. To prevent that, when in
polyamorous mode, the socket must act as a "best-effort" delivery,
dropping messages (or possibly returning them to the user with an error)
when the remote peer is unable to handle them. (A per-peer outgoing
configurable buffer <strong>MAY</strong> be used to allow some queuing to occur before
messages are dropped.)</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_best_effort_delivery">Best Effort Delivery</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Note that even in monogamous partnerships, message delivery cannot be
guaranteed as there is no acknowledgement. This is true even when the
underlying transport(s) are themselves reliable, as intermediate
routing steps may drop messages; an implementation may also fail to
deliver messages to an application after the transport message has
already been received.</p>
</div>
<div class="paragraph">
<p>Applications which need reliable delivery may build acknowledgements
on top of this protocol.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_underlying_protocol">Underlying protocol</h2>
<div class="sectionbody">
<div class="paragraph">
<p>At the transport layer, even in polyamorous mode, all of the relationships
are peer wise and unicast; therefore this protocol can be run over any
supported unicast transport.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_protocol_identity">Protocol Identity</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This protocol is assigned the protocol number <code>17</code>, which represents
major protocol number <code>1</code>, subprotocol number <code>1</code>. (The historical version
of the protocol used number <code>16</code>.)</p>
</div>
<div class="paragraph">
<p>This protocol is assigned the name <code>pair1</code>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_overview_of_the_algorithm">Overview of the Algorithm</h2>
<div class="sectionbody">
<div class="paragraph">
<p>A <em>PAIR</em> endpoint can be connected only to another <em>PAIR</em> endpoint.
If some other peer attempts to connect to a <em>PAIR</em> endpoint, then
the <em>PAIR</em> endpoint <strong>MUST</strong> close the underlying transport channel.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_hop_by_hop_routing">Hop-By-Hop Routing</h2>
<div class="sectionbody">
<div class="paragraph">
<p>When creating more complex topologies, PAIR endpoints are
paired in the intermediate nodes to form a forwarding component;
we call this component a <em>device</em>. Multiple <em>devices</em> can be
chained together.</p>
</div>
<div class="imageblock">
<div class="content">
<img src="pairv1-hopbyhop.png" alt="pairv1 hopbyhop" width="520" height="126">
</div>
</div>
<div class="paragraph">
<p>In this way, complex topologies can be built, and forwarding
chains can be created to allow messages to pass through complex
underlying transport boundaries in a manner that is transparent
to the application.</p>
</div>
<div class="paragraph">
<p>Each peer need only know it’s own nearest partner(s).</p>
</div>
<div class="paragraph">
<p>It is also possible to create <em>device</em> implementations that
support polyamorous relationships. In this case, the <em>device</em>
must decide to which of several partners a message should be
forwarded.</p>
</div>
<div class="paragraph">
<p>Given the above, it is apparent that in the general case
a polyamorous <em>device</em> must retain enough information to
ensure that once two remote peers are connected, the same
peers are used for further communication. This is necessary
as there may be implied state in the messages sent and
received, such that a given peer may not expect or tolerate
a change in with which remote peer it is communication.</p>
</div>
<div class="paragraph">
<p>It is possible for implementations to have more detailed knowledge
of the application protocol(s) in use above this <em>PAIR</em> protocol.
In such a case, a <em>device</em> implementation <strong>MAY</strong> forward a message
however it chooses.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_peer_selection">Peer Selection</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Polyamorous implementations <strong>MUST</strong> provide a means for polyamorous
applications to choose to which particular remote peer a message
shall be sent. Monogamous implementation <strong>MAY</strong> offer this ability.</p>
</div>
<div class="paragraph">
<p>In either case, if a remote peer is specified, the message <strong>MUST</strong>
be delivered to that peer if possible; it <strong>MUST NOT</strong> be delivered
to any other peer. In such a case implementations <strong>SHOULD</strong> provide
an indication of a failure to send to the application.</p>
</div>
<div class="paragraph">
<p>Polyamorous implementations <strong>MUST</strong> offer the ability for applications
to determine from which remote peer a message was received, in the same
format that would be used for sending a message to the same peer.</p>
</div>
<div class="paragraph">
<p>If no peer is specified when sending a message, then the implementation
<strong>MAY</strong> choose any peer at it’s discretion. Implementations <strong>SHOULD</strong> default
to sending to the same peer when none is specified. (As an exception,
when an implementation knows that the upper protocol and applications
are stateless, then if no peer is specified they <strong>MAY</strong> choose a remote peer
at their own discretion.)</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_loop_avoidance">Loop Avoidance</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In order to prevent the creation of forwarding loops, a hop-count is
added to the message header, allowing implementations to determine
through how many intermediate <em>device</em> nodes a message has passed.</p>
</div>
<div class="paragraph">
<p>Implementations <strong>MUST</strong> have a configurable maximum hop-count, and <strong>MUST</strong>
discard any message which has exceeded it’s hop count. Implementations
<strong>SHOULD NOT</strong> disconnect channels underneath though, since a given device
peer may be polyamorous and disconnecting would potentially impact
peers that would otherwise be unaffected.</p>
</div>
<div class="paragraph">
<p>The default limit for hops <strong>SHALL</strong> be 8.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_per_message_header">Per Message Header</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Each message is prefixed with a 32-bit header. The header consists
of the following:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="pairv1-header.png" alt="pairv1 header" width="896" height="192">
</div>
</div>
<div class="paragraph">
<p>This is a 32-bit word (big-endian). The upper 24 bits <strong>MUST</strong> be zero.
The lower 8 bits contain <strong>SHALL</strong> current hop count.</p>
</div>
<div class="paragraph">
<p>Each time the message is sent, the current hop count (which starts at 0)
<strong>SHALL</strong> be incremented.</p>
</div>
<div class="paragraph">
<p>Therefore every message transmitted shall have a hop count of at least 1.
When this value exceeds the implementation defined hop limit (see
<a href="#_loop_avoidance">Loop Avoidance</a>), it is discarded.</p>
</div>
<div class="paragraph">
<p>Implementations <strong>MUST</strong> discard any message with a hop count of zero, since
that may represent a wrap from 255, as well as any message where the
reserved bits are not zero.</p>
</div>
</div>
</div>
</div>
<div id="footnotes">
<hr>
<div class="footnote" id="_footnote_1">
<a href="#_footnoteref_1">1</a>. That version lacked the protocol header containing the information necessary to support routing.
</div>
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<div id="footer-text">
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Last updated 2017-11-07 20:20:31 UTC
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