MeshCore Protocol Specification

Section 20: Bridge Protocol

Overview

The bridge protocol enables MeshCore packet forwarding over non-LoRa transports such as RS232 serial links and ESP-NOW WiFi. It adds a simple framing layer with a magic number and checksum around the raw MeshCore packet.

Frame Format

┌──────────────┬──────────────┬──────────────┬──────────────┐
│  Magic (2B)  │   Payload    │ Checksum (2B)│              │
│   0xC03E     │  (variable)  │ Fletcher-16  │              │
└──────────────┴──────────────┴──────────────┴──────────────┘

Fields

Field	Size	Type	Description
Magic	2 bytes	uint16	`0xC03E` — identifies the frame as a MeshCore bridge packet
Payload	variable	raw	Raw MeshCore packet (as produced by `Packet::writeTo()`)
Checksum	2 bytes	Fletcher-16	Fletcher-16 checksum over the payload

RS232 Variant

The RS232 variant adds a length field between the magic and payload:

┌──────────────┬──────────────┬──────────────┬──────────────┐
│  Magic (2B)  │  Length (2B) │   Payload    │ Checksum (2B)│
│   0xC03E     │  uint16_le   │  (variable)  │ Fletcher-16  │
└──────────────┴──────────────┴──────────────┴──────────────┘

Field	Size	Type	Description
Length	2 bytes	uint16_le	Length of the payload in bytes

Fletcher-16 Checksum

The Fletcher-16 checksum is computed over the payload bytes:

function fletcher16(data):
    sum1 = 0
    sum2 = 0
    for byte in data:
        sum1 = (sum1 + byte) mod 255
        sum2 = (sum2 + sum1) mod 255
    return (sum2 << 8) | sum1

ESP-NOW Variant

For ESP-NOW transport, the frame format is the same as the basic format (magic + payload + checksum) without the length field, since ESP-NOW provides its own length framing.

Cross-References

Section 1: Wire Format — Packet format within payload

Reference Implementation

Bridge handling in MeshCore firmware (board-specific implementations)

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MeshCore Protocol Specification — v0.1.0