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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 1.2,</span> |
| <span id="revdate">2017-11-07</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?style=for-the-badge" 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: 1.2</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="_monogamous_mode">Monogamous Mode</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p><em>Monogamous</em> mode peers only communicate with at most one other peer. |
| Furthermore, they are <em>strictly</em> monogamous, in that they will reject |
| attempts to form additional relationships.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Accordingly, when another peer attempts to establish communication |
| with a monogamous peer, that peer <strong>MUST</strong> drop the connection or |
| perform other transport-specific appropriate actions to reject |
| the connection.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Because there is exactly one other party with him the peer is communicating, |
| there is no need for applications to specify the destination of |
| the message.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Implementations <strong>MUST</strong> support monogamous mode.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_polyamorous_mode">Polyamorous Mode</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p><em>Polyamorous</em> mode peers are willing to establish multiple relationships |
| with peers, and communicate amongst them. However applications must be |
| aware of this mode of operation. Implementation of polyamorous mode |
| is <strong>OPTIONAL</strong>.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_peer_selection">Peer Selection</h3> |
| <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.</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> |
| <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> |
| </div> |
| <div id="footer"> |
| <div id="footer-text"> |
| Version 1.2<br> |
| Last updated 2017-11-27 18:05:44 UTC |
| </div> |
| </div> |
| </body> |
| </html> |
