MeshCore Protocol Specification

Section 12: Payload — Control

Overview

The CONTROL payload carries control and discovery data. Control packets are raw byte payloads whose structure is determined by the first byte. A subset of control packets (those with bit 7 of the first byte set) are restricted to zero-hop delivery only.

Payload Type

Header payload type field: 0x0B (CONTROL)

Wire Format

 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Control Byte |  Data...      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields

Zero-Hop Restriction

When bit 7 (0x80) of the control byte is set, the packet is restricted to zero-hop delivery only. The reference implementation enforces this:

• If (payload[0] & 0x80) != 0 AND the packet is direct-routed:
  • It is processed only if getPathHashCount() == 0.
  • It is NOT forwarded to other nodes.

Discovery Protocols

Control packets are commonly used for node discovery. The specific sub-protocol formats are application-defined, but typically include:

• Discovery Request: Sent zero-hop to find nearby nodes
• Discovery Response: Contains node type, name, SNR, and other metadata

Encoding

1. Construct the raw control data bytes.
2. If the control packet should be zero-hop only, set bit 7 of the first byte.
3. Copy data to payload, set payload_len.

Constraints

• Payload MUST be at least 1 byte (the control byte).
• Maximum payload: 184 bytes (MAX_PACKET_PAYLOAD).

Cross-References

• Section 17: Routing — Zero-hop delivery
• Test vectors: corpus/payloads/control/

Reference Implementation

• Mesh::createControlData() in src/Mesh.cpp — Encoding
• Mesh::onRecvPacket(), CONTROL handling — Zero-hop check and delivery