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<title>ZeroTier Mapping for Scalability Protocols</title>
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<body class="article">
<div id="header">
<h1>ZeroTier Mapping for Scalability Protocols</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 0.8,</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="_zerotier_mapping_for_scalability_protocols">ZeroTier Mapping for Scalability Protocols</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&#8217;Amore</a></p>
</li>
<li>
<p>Version 0.8</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_abstract">Abstract</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This document defines the
<a href="http://www.zerotier.com">ZeroTier</a> mapping for scalability protocols.
This enables SP protocols to run over a ZeroTier network.  The transport
defined here sits on top of an unreliable virtual Layer 2 transport,
and does not require a TCP/IP stack.</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&#8217;Amore</a><br>
Copyright 2017 <a href="mailto:info@capitar.com">Capitar IT Group BV</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="_underlying_protocol">Underlying protocol</h2>
<div class="sectionbody">
<div class="paragraph">
<p>ZeroTier expresses an 802.3 style layer 2, where frames maybe exchanged as if
they were Ethernet frames.  Virtual broadcast domains are created within a
numbered "network", and frames may then be exchanged with any peers on that
network.</p>
</div>
<div class="paragraph">
<p>Frames may arrive in any order, or be lost, just a with Ethernet
(best effort delivery), but they are strongly protected by a
cryptographic checksum, so frames that do arrive will be uncorrupted.
Furthermore, ZeroTier guarantees that a given frame will be received
at most once.</p>
</div>
<div class="paragraph">
<p>Each application on a ZeroTier network has its own address<sup class="footnote">[<a id="_footnoteref_1" class="footnote" href="#_footnote_1" title="View footnote.">1</a>]</sup>,
called a
ZeroTier ID (<em>ZTID</em>), which is globally unique&#8201;&#8212;&#8201;this is generated
from a hash of the public key associated with the application.</p>
</div>
<div class="paragraph">
<p>A given application may participate in multiple ZeroTier networks.</p>
</div>
<div class="paragraph">
<p>Sharing of ZeroTier IDs between applications, as well as use of multiple
_ZTID_s within a single application, as well as management of the associated
ZeroTier-specific state is out of scope for this document.</p>
</div>
<div class="paragraph">
<p>ZeroTier networks have a standard MTU of 2800 bytes, but over typical
public networks an "optimum" MTU of 1400 bytes is used.
ZeroTier may be configured to have larger MTUs, but typically this involves
extensive reassembly at underlying layers, and implementations <strong>SHOULD</strong>
use the optimum MTU advertised by the ZeroTier implementation.</p>
</div>
<div class="paragraph">
<p>Note that at this time, broadcast and multicast is not supported by
this mapping.  (A future update may resolve this.)</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_packet_layout">Packet layout</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Each SP message sent over ZeroTier is comprised of one or
more fragments, where each fragment is mapped to a single underlying
ZeroTier L2 frame.  We use the EtherType field of 0x0901 to indicate
SP over ZeroTier protocol (number to be registered with IEEE).</p>
</div>
<div class="paragraph">
<p>The ZeroTier L2 payload shall be encoded with a header as follows:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="zerotier0-header.png" alt="zerotier0 header" width="896" height="360">
</div>
</div>
<div class="paragraph">
<p>All numeric fields are in big-endian byte order.  Note that ZeroTier
APIs present this as the L2 payload, but ZeroTier itself may prepend
additional data such as the Ethernet type, and source and destination
MAC addresses, as well as ZeroTier specific headers.  The details of
such headers are out of scope for this document.</p>
</div>
<div class="paragraph">
<p>As above, the start of each frame is just as a normal Ethernet payload.
The Ethernet type (ethertype) we use for these frames is 0x901, with
a VLAN ID of 0.</p>
</div>
<div class="paragraph">
<p>The <code>op</code> is a field that indicates the type of message being sent.  The
following values are defined:</p>
</div>
<table class="tableblock frame-all grid-all spread">
<colgroup>
<col style="width: 20%;">
<col style="width: 80%;">
</colgroup>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x00</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="#_data_messages"><code>DATA</code></a></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x10</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="#_conn_req_and_conn_ack_messages"><code>CONN-REQ</code></a></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x12</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="#_conn_req_and_conn_ack_messages"><code>CONN-ACK</code></a></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x20</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="#_disc_messages"><code>DISC</code></a></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x30</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="#_ping_and_pong_messages"><code>PING</code></a></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x32</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="#_ping_and_pong_messages"><code>PONG</code></a></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x40</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><a href="#_err_messages"><code>ERR</code></a></p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>These are discussed further below.  Implementations
<strong>MUST</strong> discard messages where the <code>op</code> is not one of these.</p>
</div>
<div class="paragraph">
<p>The <code>flags</code> field is reserved for future use, and <strong>MUST</strong> be zero.
Implementations <strong>MUST</strong> discard frames for which this is not true.</p>
</div>
<div class="paragraph">
<p>The <code>version</code> byte <strong>MUST</strong> be set to <code>0x1</code>.  Implementations <strong>MUST</strong> discard
any messages received for any other version.</p>
</div>
<div class="paragraph">
<p>The <code>source port</code> and <code>destination port</code> are used to construct a logical
conversation.  These are 24-bits wide, and are discussed further below.
The <code>reserved</code> fields must be set to zero.</p>
</div>
<div class="paragraph">
<p>The remainder of frame varies depending on the <code>op</code> used.</p>
</div>
<div class="paragraph">
<p>Note that it is not by accident that the payload is 32-bit aligned in
this message format.  The payload is actually 64-bit aligned.</p>
</div>
<div class="sect2">
<h3 id="_port_fields">Port Fields</h3>
<div class="paragraph">
<p>The port fields are used to discriminate different uses, allowing one
application to have multiple connections or sockets open.  The
purpose is analogous to TCP port numbers, except that instead of the
operating system performing the discrimination the application or
library code must do so.  Note that port numbers are 24-bits.  This
was chosen to allow a peer to allocate a unique port number for each
local conversation, allowing up to 16 million concurrent conversations.
This also allows a 40-bit node number to be combined with the 24-bit
port number to create a 64-bit unique address.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_data_messages">DATA messages</h2>
<div class="sectionbody">
<div class="paragraph">
<p><code>DATA</code> messages carry SP protocol payload data.  They can only be sent
on an established session (see <code>CONN</code> messages below), and are never
acknowledged (in this version).  The op-specific payload they carry
is formed like this:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="zerotier0-data.png" alt="zerotier0 data" width="896" height="240">
</div>
</div>
<div class="paragraph">
<p>All fragments, except for the last, <strong>MUST</strong> be the same size.  The fragment
size field carries the size of every fragment, except that the last
fragment may be shorter; however even for the last fragment, the fragment
size <strong>MUST</strong> be the size of the rest of the fragments.  This is necessary
to allow a receiver to know the fragment size of the other fragments even
if the final fragment is received before any others.  (Typically this may
occur if a message consisting of two fragments arrives with fragments
out of order.)</p>
</div>
<div class="paragraph">
<p>The last fragment shall have the fragment number equal to
the total fragments minus one, and the first fragment shall have fragment
number 0.  Under typical optimal conditions, with an optimal MTU of 1400
bytes, the largest message that can be transmitted is approximately 86 MB.
Specifically the limit is (65534 * (1400 - 20)) = 90,436,920 bytes.
(Larger MTUs may be used, if the implementation determines that it is
advantageous to do so.  Doing so would necessarily give a larger maximum
message size.)</p>
</div>
<div class="paragraph">
<p>However, transmitting such a large message would require sending over
65 thousand fragments, and given the likelihood of fragment loss, and
the lack of acknowledgment, it is likely that the entire message would
be lost.  As a result, implementations are encouraged to limit the
amount of data that they send to at most a few megabytes.  Implementations
receiving the first fragment can easily calculate the worst case for
the message size (the size of the user payload multiplied by the total
number of fragments), and <strong>MAY</strong> reply to the sender with an <code>ERR</code> message
using the code 0x05, indicating that the message is larger than the
receiver is willing to accept.</p>
</div>
<div class="paragraph">
<p>Each fragment for a given message must carry the same <code>message ID</code>.
Implementations <strong>MUST</strong> initialize this to a random value when starting
a conversation, and <strong>MUST</strong> increment this each time a new message is sent.
Message IDs of zero are not permitted; implementations <strong>MUST</strong> skip past zero
when incrementing message IDs.</p>
</div>
<div class="paragraph">
<p>Implementations may detect the loss of a message by noticing skips in the
message IDs that are received, accounting for the expected skip past zero.</p>
</div>
<div class="paragraph">
<p>Note that no field conveys the length of the fragment itself, as
this can be determined from the L2 length&#8201;&#8212;&#8201;the user data within
the fragment extends to the end of the L2 payload supplied by ZeroTier.
(And, all fragments other than the final fragment for a message must
therefore have the same length.)</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_conn_req_and_conn_ack_messages">CONN-REQ and CONN-ACK messages</h2>
<div class="sectionbody">
<div class="paragraph">
<p><code>CONN-REQ</code> frames represent a request from an initiator to establish a
session, i.e. a new conversation or connection, and <code>CONN-ACK</code>
messages are the normal successful reply from the responder.  They both
take the same form, which consists of the usual headers along with the
senders 16-bit (big-endian) SP protocol ID appended:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="zerotier0-conn.png" alt="zerotier0 conn" width="536" height="160">
</div>
</div>
<div class="paragraph">
<p>The connection is initiated by the initiator sending this message,
with its own SP protocol ID, with the <code>op</code> set to <code>CONN-REQ</code>.
The initiator must choose a <code>source port</code> number that is not currently
being used with the remote peer. (Most implementations will choose a
a source port that is not used at all. Source port numbers <strong>SHOULD</strong>
be chosen randomly.)</p>
</div>
<div class="paragraph">
<p>The responder will acknowledge this by replying with its SP protocol
ID in the 4-byte payload, using the <code>CONN-ACK</code> op.  Additionally,
the source port number that the responder replies with <strong>MUST</strong> be the
one the intiator requested.</p>
</div>
<div class="paragraph">
<p>(Responders will identify the session using the initiators chosen
<code>source port</code>, which the initiator <strong>MUST NOT</strong> concurrently use for any
other sessions.)</p>
</div>
<div class="paragraph">
<p>Alternatively, a responder <strong>MAY</strong> reject the connection attempt by
sending a suitably formed ERR message (see below).</p>
</div>
<div class="paragraph">
<p>If a sender does not receive a reply, it <strong>SHOULD</strong> retry this message
before giving up and reporting an error to the user.  It is recommended
that a configurable number of retries and time interval be used.</p>
</div>
<div class="paragraph">
<p>Given modern Internet latencies of generally less than 500 ms, resending
up to 12 <code>CONN-REQ</code> requests, once every 5 seconds, before giving up seems
reasonable.  (These times are somewhat larger to allow for ZeroTier
path discovery to take place; this results in a timeout of approximately
a minute.)</p>
</div>
<div class="paragraph">
<p>The initiator <strong>MUST NOT</strong> send any <code>DATA</code> messages for a conversation until
it has received an ACK from the other party, and it <strong>MUST</strong> send all further
messages for the conversation to the port number supplied by the responder.</p>
</div>
<div class="paragraph">
<p>If a <code>CONN-REQ</code> frame is received by a responder for a conversation that already
exists, the responder <strong>MUST</strong> reply.  Further, the source port it replies with,
and the SP protocol IDs <strong>MUST</strong> be identical to what it first sent.  This
ensures that the <code>CONN-REQ</code> request is idempotent.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_disc_messages">DISC messages</h2>
<div class="sectionbody">
<div class="paragraph">
<p>DISC messages are used to request a session be terminated.  This
notifies the remote sender that no more data will be sent or
accepted, and the session resources may be released.  There is no
payload. There is no acknowledgment.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_ping_and_pong_messages">PING and PONG messages</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In order to keep session state, implementations will generally store
data for each session.  In order to prevent a stale session from
consuming these resources forever, and in order to keep underlying
ZeroTier sessions alive, a <code>PING</code> message <strong>MAY</strong> be sent to a peer
with whom a session has been established.  This message has no payload.</p>
</div>
<div class="paragraph">
<p>If the <code>PING</code> is is successful, then the responder <strong>MUST</strong> reply with a <code>PONG</code>
message.  As with <code>PING</code>, the <code>PONG</code> message carries no payload.</p>
</div>
<div class="paragraph">
<p>There is no response to a <code>PONG</code> message.</p>
</div>
<div class="paragraph">
<p>In the event of an error, an implementation <strong>MAY</strong> reply with an <code>ERR</code>
message.</p>
</div>
<div class="paragraph">
<p>Implementations <strong>SHOULD NOT</strong> initiate <code>PING</code> messages if they have either
received other session messages recently.</p>
</div>
<div class="paragraph">
<p>Implementations <strong>SHOULD</strong> use a timeout <em>T1</em> seconds of be used before
initiating a message the first time, and that in the absence of a
reply, up to <em>N</em> further attempts be made, separated by <em>T2</em> seconds.  If
no reply to the <em>N_th attempt is received after _T2</em> seconds have passed,
then the remote peer should be assumed offline or dead, and the
session closed.</p>
</div>
<div class="paragraph">
<p>The values for <em>T1</em>, <em>T2</em>, and <em>N</em> <strong>SHOULD</strong> be configurable, with
recommended default values of 60, 10, and 5.  With these values,
sessions that appear dead after 2 minutes will be closed, and their
resources reclaimed.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_err_messages">ERR messages</h2>
<div class="sectionbody">
<div class="paragraph">
<p><code>ERR</code> messages indicate a failure in the session, and abruptly
terminate the session.  The payload for these messages consists of a
single byte error code, followed by an ASCII message describing the
error (not terminated by zero).  This message <strong>MUST NOT</strong> be more than
128 bytes in length.</p>
</div>
<div class="paragraph">
<p>The following error codes are defined:</p>
</div>
<table class="tableblock frame-all grid-all spread">
<colgroup>
<col style="width: 20%;">
<col style="width: 80%;">
</colgroup>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x01</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">No party listening at that address or port.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x02</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">No such session found.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x03</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">SP protocol ID invalid.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x04</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Generic protocol error.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0x05</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Message size too big.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">0xff</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Other uncategorized error.</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>Implementations <strong>MUST</strong> discard any session state upon receiving an <code>ERR</code>
message.  These messages are not acknowledged.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_message_reassembly">Message Reassembly</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Implementations <strong>MUST</strong> accept and reassemble fragmented <code>DATA</code> messages.
Implementations <strong>MUST</strong> discard fragmented messages of other types.</p>
</div>
<div class="paragraph">
<p>Messages larger than the ZeroTier MTU <strong>MUST</strong> be fragmented.</p>
</div>
<div class="paragraph">
<p>Implementations SHOULD limit the number of unassembled messages
retained for reassembly, to minimize the likelihood of intentional
abuse.  It is suggested that at most 2 unassembled messages be
retained.  It is further suggested that if 2 or more unfragmented
messages arrive before a message is reassembled, or more than 5
seconds pass before the reassembly is complete, that the unassembled
fragments be discarded.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_ports">Ports</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The port numbers are 24-bit fields, allowing a single <em>ZTID</em> to
service multiple application layer protocols, which could be treated
as separate end points, or as separate sockets in the application.
The implementation is responsible for discriminating on these and
delivering to the appropriate consumer.</p>
</div>
<div class="paragraph">
<p>As with UDP or TCP, it is intended that each party have its own port
number, and that a pair of ports (combined with ZeroTier IDs) be used
to identify a single conversation.</p>
</div>
<div class="paragraph">
<p>An SP server <strong>SHOULD</strong> allocate a port for number advertisement.  It is
expected clients will generate ephemeral port numbers.</p>
</div>
<div class="paragraph">
<p>Implementations are free to choose how to allocate port numbers, but
it is <strong>RECOMMENDED</strong> that administratively configured port numbers are small,
with the high order bit clear, and that numbers larger than 2<sup>23</sup> (high order
bit set) be used for ephemeral allocations.</p>
</div>
<div class="paragraph">
<p>It is <strong>RECOMMENDED</strong> that separate short queues (perhaps just one or two
messages long) be kept per local port in implementations, to prevent
head-of-line blocking issues where backpressure on one consumer
(perhaps just a single thread or socket) blocks others.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_uri_format">URI Format</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The URI scheme used to represent ZeroTier addresses makes use of
ZeroTier IDs, ZeroTier network IDs, and our own 24-bit ports.</p>
</div>
<div class="paragraph">
<p>The format <strong>SHALL</strong> be <code>zt://<em>nwid</em>/<em>ztid:port</em></code>, where the <code><em>nwid</em></code>
component represents the 64-bit hexadecimal ZeroTier network ID,
the <code><em>ztid</em></code> represents the 40-bit hexadecimal ZeroTier Device ID,
and the <code><em>port</em></code> is the 24-bit port number (decimal) previously described.</p>
</div>
<div class="paragraph">
<p>A responder <strong>MAY</strong> elide the <code>/<em>ztid</em></code> portion, to just bind to itself,
in which case the format will be <code>zt://<em>nwid:port</em></code>.</p>
</div>
<div class="paragraph">
<p>A port number of 0 <strong>MAY</strong> be used when listening to indicate that a random
ephemeral port should be chosen.</p>
</div>
<div class="paragraph">
<p>An implementation <strong>MAY</strong> allow the <code><em>ztid</em></code> 0 be replaced with <code>*</code> to
indicate that the node&#8217;s local <em>ZTID</em> be used.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_security_considerations">Security Considerations</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The mapping isn&#8217;t intended to provide any additional security beyond that
provided by ZeroTier itself.  Managing the key materials used by ZeroTier
is implementation-specific, and they must take the appropriate care when
dealing with them.</p>
</div>
</div>
</div>
</div>
<div id="footnotes">
<hr>
<div class="footnote" id="_footnote_1">
<a href="#_footnoteref_1">1</a>. Technically an application may have more than one ZeroTier address, but such uses are unusual.
</div>
</div>
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<div id="footer-text">
Version 0.8<br>
Last updated 2017-11-27 18:05:44 UTC
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