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<h1 id="packet-format">Packet Format</h1>
<p>This document describes the MeshCore packet format.</p>
<ul>
<li><code>0xYY</code> indicates <code>YY</code> in hex notation.</li>
<li><code>0bYY</code> indicates <code>YY</code> in binary notation.</li>
<li>Bit 0 indicates the bit furthest to the right: <code>0000000X</code></li>
<li>Bit 7 indicates the bit furthest to the left: <code>X0000000</code></li>
</ul>
<h2 id="version-1-packet-format">Version 1 Packet Format</h2>
<p>This is the protocol level packet structure used in MeshCore firmware v1.12.0</p>
<pre><code>[header][transport_codes(optional)][path_length][path][payload]
</code></pre>
<ul>
<li><a href="#header-format">header</a> - 1 byte<ul>
<li>8-bit Format: <code>0bVVPPPPRR</code> - <code>V=Version</code> - <code>P=PayloadType</code> - <code>R=RouteType</code></li>
<li>Bits 0-1 - 2-bits - <a href="#route-types">Route Type</a><ul>
<li><code>0x00</code>/<code>0b00</code> - <code>ROUTE_TYPE_TRANSPORT_FLOOD</code> - Flood Routing + Transport Codes</li>
<li><code>0x01</code>/<code>0b01</code> - <code>ROUTE_TYPE_FLOOD</code> - Flood Routing</li>
<li><code>0x02</code>/<code>0b10</code> - <code>ROUTE_TYPE_DIRECT</code> - Direct Routing</li>
<li><code>0x03</code>/<code>0b11</code> - <code>ROUTE_TYPE_TRANSPORT_DIRECT</code> - Direct Routing + Transport Codes</li>
</ul>
</li>
<li>Bits 2-5 - 4-bits - <a href="#payload-types">Payload Type</a><ul>
<li><code>0x00</code>/<code>0b0000</code> - <code>PAYLOAD_TYPE_REQ</code> - Request (destination/source hashes + MAC)</li>
<li><code>0x01</code>/<code>0b0001</code> - <code>PAYLOAD_TYPE_RESPONSE</code> - Response to <code>REQ</code> or <code>ANON_REQ</code></li>
<li><code>0x02</code>/<code>0b0010</code> - <code>PAYLOAD_TYPE_TXT_MSG</code> - Plain text message</li>
<li><code>0x03</code>/<code>0b0011</code> - <code>PAYLOAD_TYPE_ACK</code> - Acknowledgment</li>
<li><code>0x04</code>/<code>0b0100</code> - <code>PAYLOAD_TYPE_ADVERT</code> - Node advertisement</li>
<li><code>0x05</code>/<code>0b0101</code> - <code>PAYLOAD_TYPE_GRP_TXT</code> - Group text message (unverified)</li>
<li><code>0x06</code>/<code>0b0110</code> - <code>PAYLOAD_TYPE_GRP_DATA</code> - Group datagram (unverified)</li>
<li><code>0x07</code>/<code>0b0111</code> - <code>PAYLOAD_TYPE_ANON_REQ</code> - Anonymous request</li>
<li><code>0x08</code>/<code>0b1000</code> - <code>PAYLOAD_TYPE_PATH</code> - Returned path</li>
<li><code>0x09</code>/<code>0b1001</code> - <code>PAYLOAD_TYPE_TRACE</code> - Trace a path, collecting SNR for each hop</li>
<li><code>0x0A</code>/<code>0b1010</code> - <code>PAYLOAD_TYPE_MULTIPART</code> - Packet is part of a sequence of packets</li>
<li><code>0x0B</code>/<code>0b1011</code> - <code>PAYLOAD_TYPE_CONTROL</code> - Control packet data (unencrypted)</li>
<li><code>0x0C</code>/<code>0b1100</code> - reserved</li>
<li><code>0x0D</code>/<code>0b1101</code> - reserved</li>
<li><code>0x0E</code>/<code>0b1110</code> - reserved</li>
<li><code>0x0F</code>/<code>0b1111</code> - <code>PAYLOAD_TYPE_RAW_CUSTOM</code> - Custom packet (raw bytes, custom encryption)</li>
</ul>
</li>
<li>Bits 6-7 - 2-bits - <a href="#payload-versions">Payload Version</a><ul>
<li><code>0x00</code>/<code>0b00</code> - v1 - 1-byte src/dest hashes, 2-byte MAC</li>
<li><code>0x01</code>/<code>0b01</code> - v2 - Future version (e.g., 2-byte hashes, 4-byte MAC)</li>
<li><code>0x02</code>/<code>0b10</code> - v3 - Future version</li>
<li><code>0x03</code>/<code>0b11</code> - v4 - Future version</li>
</ul>
</li>
</ul>
</li>
<li><code>transport_codes</code> - 4 bytes (optional)<ul>
<li>Only present for <code>ROUTE_TYPE_TRANSPORT_FLOOD</code> and <code>ROUTE_TYPE_TRANSPORT_DIRECT</code></li>
<li><code>transport_code_1</code> - 2 bytes - <code>uint16_t</code> - calculated from region scope</li>
<li><code>transport_code_2</code> - 2 bytes - <code>uint16_t</code> - reserved</li>
</ul>
</li>
<li><code>path_length</code> - 1 byte - Encoded path metadata<ul>
<li>Bits 0-5 store path hash count / hop count (<code>0-63</code>)</li>
<li>Bits 6-7 store path hash size minus 1<ul>
<li><code>0b00</code>: 1-byte path hashes</li>
<li><code>0b01</code>: 2-byte path hashes</li>
<li><code>0b10</code>: 3-byte path hashes</li>
<li><code>0b11</code>: reserved / unsupported</li>
</ul>
</li>
</ul>
</li>
<li><code>path</code> - <code>hop_count * hash_size</code> bytes - Path to use for Direct Routing or flood path tracking<ul>
<li>Up to a maximum of 64 bytes, defined by <code>MAX_PATH_SIZE</code></li>
<li>Effective byte length is calculated from the encoded hop count and hash size, not taken directly from <code>path_length</code></li>
<li>v1.12.0 firmware and older only handled legacy 1-byte path hashes and dropped packets whose path bytes exceeded <a href="https://github.com/meshcore-dev/MeshCore/blob/e812632235274ffd2382adf5354168aec765d416/src/Dispatcher.cpp#L144">64 bytes</a></li>
</ul>
</li>
<li><code>payload</code> - variable length - Payload Data<ul>
<li>Up to a maximum 184 bytes, defined by <code>MAX_PACKET_PAYLOAD</code></li>
<li>Generally this is the remainder of the raw packet data</li>
<li>The firmware parses this data based on the provided Payload Type</li>
<li>v1.12.0 firmware and older drops packets with <code>payload</code> sizes <a href="https://github.com/meshcore-dev/MeshCore/blob/e812632235274ffd2382adf5354168aec765d416/src/Dispatcher.cpp#L152">larger than 184</a></li>
</ul>
</li>
</ul>
<h3 id="packet-format_1">Packet Format</h3>
<table>
<thead>
<tr>
<th>Field</th>
<th>Size (bytes)</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>header</td>
<td>1</td>
<td>Contains routing type, payload type, and payload version</td>
</tr>
<tr>
<td>transport_codes</td>
<td>4 (optional)</td>
<td>2x 16-bit transport codes (if ROUTE_TYPE_TRANSPORT_*)</td>
</tr>
<tr>
<td>path_length</td>
<td>1</td>
<td>Encodes path hash size in bits 6-7 and hop count in bits 0-5</td>
</tr>
<tr>
<td>path</td>
<td>up to 64 (<code>MAX_PATH_SIZE</code>)</td>
<td>Stores <code>hop_count * hash_size</code> bytes of path data if applicable</td>
</tr>
<tr>
<td>payload</td>
<td>up to 184 (<code>MAX_PACKET_PAYLOAD</code>)</td>
<td>Data for the provided Payload Type</td>
</tr>
</tbody>
</table>
<blockquote>
<p>NOTE: see the <a href="../payloads/">Payloads</a> documentation for more information about the content of specific payload types.</p>
</blockquote>
<h3 id="header-format">Header Format</h3>
<p>Bit 0 means the lowest bit (1s place)</p>
<table>
<thead>
<tr>
<th>Bits</th>
<th>Mask</th>
<th>Field</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>0-1</td>
<td><code>0x03</code></td>
<td>Route Type</td>
<td>Flood, Direct, etc</td>
</tr>
<tr>
<td>2-5</td>
<td><code>0x3C</code></td>
<td>Payload Type</td>
<td>Request, Response, ACK, etc</td>
</tr>
<tr>
<td>6-7</td>
<td><code>0xC0</code></td>
<td>Payload Version</td>
<td>Versioning of the payload format</td>
</tr>
</tbody>
</table>
<h3 id="route-types">Route Types</h3>
<table>
<thead>
<tr>
<th>Value</th>
<th>Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>0x00</code></td>
<td><code>ROUTE_TYPE_TRANSPORT_FLOOD</code></td>
<td>Flood Routing + Transport Codes</td>
</tr>
<tr>
<td><code>0x01</code></td>
<td><code>ROUTE_TYPE_FLOOD</code></td>
<td>Flood Routing</td>
</tr>
<tr>
<td><code>0x02</code></td>
<td><code>ROUTE_TYPE_DIRECT</code></td>
<td>Direct Routing</td>
</tr>
<tr>
<td><code>0x03</code></td>
<td><code>ROUTE_TYPE_TRANSPORT_DIRECT</code></td>
<td>Direct Routing + Transport Codes</td>
</tr>
</tbody>
</table>
<h3 id="path-length-encoding">Path Length Encoding</h3>
<p><code>path_length</code> is not a raw byte count. It packs both hash size and hop count:</p>
<table>
<thead>
<tr>
<th>Bits</th>
<th>Field</th>
<th>Meaning</th>
</tr>
</thead>
<tbody>
<tr>
<td>0-5</td>
<td>Hop Count</td>
<td>Number of path hashes (<code>0-63</code>)</td>
</tr>
<tr>
<td>6-7</td>
<td>Hash Size Code</td>
<td>Stored as <code>hash_size - 1</code></td>
</tr>
</tbody>
</table>
<p>Hash size codes:</p>
<table>
<thead>
<tr>
<th>Bits 6-7</th>
<th>Hash Size</th>
<th>Notes</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>0b00</code></td>
<td>1 byte</td>
<td>Legacy / default mode</td>
</tr>
<tr>
<td><code>0b01</code></td>
<td>2 bytes</td>
<td>Supported in current firmware</td>
</tr>
<tr>
<td><code>0b10</code></td>
<td>3 bytes</td>
<td>Supported in current firmware</td>
</tr>
<tr>
<td><code>0b11</code></td>
<td>4 bytes</td>
<td>Reserved / invalid</td>
</tr>
</tbody>
</table>
<p>Examples:</p>
<ul>
<li><code>0x00</code>: zero-hop packet, no path bytes</li>
<li><code>0x05</code>: 5 hops using 1-byte hashes, so path is 5 bytes</li>
<li><code>0x45</code>: 5 hops using 2-byte hashes, so path is 10 bytes</li>
<li><code>0x8A</code>: 10 hops using 3-byte hashes, so path is 30 bytes</li>
</ul>
<h3 id="payload-types">Payload Types</h3>
<table>
<thead>
<tr>
<th>Value</th>
<th>Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>0x00</code></td>
<td><code>PAYLOAD_TYPE_REQ</code></td>
<td>Request (destination/source hashes + MAC)</td>
</tr>
<tr>
<td><code>0x01</code></td>
<td><code>PAYLOAD_TYPE_RESPONSE</code></td>
<td>Response to <code>REQ</code> or <code>ANON_REQ</code></td>
</tr>
<tr>
<td><code>0x02</code></td>
<td><code>PAYLOAD_TYPE_TXT_MSG</code></td>
<td>Plain text message</td>
</tr>
<tr>
<td><code>0x03</code></td>
<td><code>PAYLOAD_TYPE_ACK</code></td>
<td>Acknowledgment</td>
</tr>
<tr>
<td><code>0x04</code></td>
<td><code>PAYLOAD_TYPE_ADVERT</code></td>
<td>Node advertisement</td>
</tr>
<tr>
<td><code>0x05</code></td>
<td><code>PAYLOAD_TYPE_GRP_TXT</code></td>
<td>Group text message (unverified)</td>
</tr>
<tr>
<td><code>0x06</code></td>
<td><code>PAYLOAD_TYPE_GRP_DATA</code></td>
<td>Group datagram (unverified)</td>
</tr>
<tr>
<td><code>0x07</code></td>
<td><code>PAYLOAD_TYPE_ANON_REQ</code></td>
<td>Anonymous request</td>
</tr>
<tr>
<td><code>0x08</code></td>
<td><code>PAYLOAD_TYPE_PATH</code></td>
<td>Returned path</td>
</tr>
<tr>
<td><code>0x09</code></td>
<td><code>PAYLOAD_TYPE_TRACE</code></td>
<td>Trace a path, collecting SNR for each hop</td>
</tr>
<tr>
<td><code>0x0A</code></td>
<td><code>PAYLOAD_TYPE_MULTIPART</code></td>
<td>Packet is part of a sequence of packets</td>
</tr>
<tr>
<td><code>0x0B</code></td>
<td><code>PAYLOAD_TYPE_CONTROL</code></td>
<td>Control packet data (unencrypted)</td>
</tr>
<tr>
<td><code>0x0C</code></td>
<td>reserved</td>
<td>reserved</td>
</tr>
<tr>
<td><code>0x0D</code></td>
<td>reserved</td>
<td>reserved</td>
</tr>
<tr>
<td><code>0x0E</code></td>
<td>reserved</td>
<td>reserved</td>
</tr>
<tr>
<td><code>0x0F</code></td>
<td><code>PAYLOAD_TYPE_RAW_CUSTOM</code></td>
<td>Custom packet (raw bytes, custom encryption)</td>
</tr>
</tbody>
</table>
<h3 id="payload-versions">Payload Versions</h3>
<table>
<thead>
<tr>
<th>Value</th>
<th>Version</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>0x00</code></td>
<td>1</td>
<td>1-byte src/dest hashes, 2-byte MAC</td>
</tr>
<tr>
<td><code>0x01</code></td>
<td>2</td>
<td>Future version (e.g., 2-byte hashes, 4-byte MAC)</td>
</tr>
<tr>
<td><code>0x02</code></td>
<td>3</td>
<td>Future version</td>
</tr>
<tr>
<td><code>0x03</code></td>
<td>4</td>
<td>Future version</td>
</tr>
</tbody>
</table>
</article>
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