ac8dg
August 13, 2020, 10:51pm
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Thanks to @geeksville and @CycloMies please look at documentation that is now provided
# Mesh broadcast algorithm
## Current algorithm
The routing protocol for Meshtastic is really quite simple (and suboptimal). It is heavily influenced by the mesh routing algorithm used in [Radiohead](https://www.airspayce.com/mikem/arduino/RadioHead/) (which was used in very early versions of this project). It has four conceptual layers.
### A note about protocol buffers
Because we want our devices to work across various vendors and implementations, we use [Protocol Buffers](https://github.com/meshtastic/Meshtastic-protobufs) pervasively. For information on how the protocol buffers are used wrt API clients see [sw-design](sw-design.md), for purposes of this document you mostly only
need to consider the MeshPacket and Subpacket message types.
### Layer 1: Non reliable zero hop messaging
This layer is conventional non-reliable lora packet transmission. The transmitted packet has the following representation on the ether:
- A 32 bit LORA preamble (to allow receiving radios to synchronize clocks and start framing). We use a longer than minimum (8 bit) preamble to maximize the amount of time the LORA receivers can stay asleep, which dramatically lowers power consumption.
After the preamble the 16 byte packet header is transmitted. This header is described directly by the PacketHeader class in the C++ source code. But indirectly it matches the first portion of the "MeshPacket" protobuf definition. But notably: this portion of the packet is sent directly as the following 16 bytes (rather than using the protobuf encoding). We do this to both save airtime and to allow receiving radio hardware the option of filtering packets before even waking the main CPU.
- to (4 bytes): the unique NodeId of the destination (or 0xffffffff for NodeNum_BROADCAST)
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