Networking Protocols
Quick Reference

Open chrome://net-export (or Firefox dev tools → Network) and watch the HTTP requests your browser makes on a single page load. You will see DNS (UDP 53), TLS (TCP 443), HTTP/2 or HTTP/3, and probably a WebSocket. Read the headers: User-Agent, Set-Cookie, Strict-Transport-Security. Goal: you can name the protocols in the request graph.

Run Mosquitto on a Pi or in Docker. Run an ESP32 sensor sketch (from the electronics quickref) that publishes a temperature reading to home/lab/temperature every 10 seconds. Subscribe with mosquitto_sub to see the data. Add TLS (mosquitto.conf + a self-signed cert or Let’s Encrypt). Goal: a working pub/sub sensor on your local network, end-to-end TLS.

Wire an RS-485 Modbus energy meter or a Modbus-enabled solar inverter to an ESP32 with a MAX485 transceiver. Poll it every 5 seconds and republish the values to MQTT. Use the Modbus cheat sheet (section 6.5) for the Python or C code. Goal: a wired industrial sensor is now in the same dashboard as your wireless sensors.

Set up RTSP with an IP camera (or your phone as a test). Add an ONVIF-capable NVR like Agent DVR or Frigate. Add a low-latency WebRTC viewer (Janus or LiveKit) so you can view the camera from anywhere with sub-second latency. Compare with HLS for higher-latency but simpler setup. Goal: video streams end-to-end and you understand the trade-offs.

Generate a real PKI with a private CA (use step ca or smallstep). Issue mTLS certs to every device in your MQTT + WebRTC + custom-HTTP stack. Set up OAuth 2.0 + OIDC with Authentik or Keycloak, and have every web UI use it. Add SSH keys everywhere, disable password login, run fail2ban. Goal: no plaintext password or self-signed cert anywhere in your stack.

The convention is to use a hierarchy: <root>/<location>/<device>/<measurement>. Wildcards: + (one level), # (all levels below).

# Examples (Home Assistant style):
home/garage/temperature          # state
home/garage/temperature/set      # command
home/barn/water_tank/level       # state
home/barn/water_tank/level/json  # JSON payload

# Wildcards
home/+/temperature              # matches any location
home/barn/#                      # matches anything under barn

# Reserved topics
$SYS/broker/version              # broker info
$SYS/clients/connected           # who's connected

# Sparkplug B industrial hierarchy:
spBv1.0/group_id/message_type/edge_node_id/device_id

# Modbus RTU over RS-485 (Python)
from pymodbus.client import ModbusSerialClient
c = ModbusSerialClient(port='/dev/ttyUSB0', baudrate=9600,
                       parity='N', stopbits=1, bytesize=8)
result = c.read_holding_registers(address=0, count=10, slave=1)
if not result.isError():
    print(result.registers)  # list of 10 uint16s

# Modbus TCP (Python)
from pymodbus.client import ModbusTcpClient
c = ModbusTcpClient('inverter.local', port=502)
result = c.read_input_registers(address=0, count=10, slave=1)

# Register types
# 0xxxxx = Coils (digital, read/write)
# 1xxxxx = Discrete inputs (digital, read-only)
# 3xxxxx = Input registers (analog, read-only)
# 4xxxxx = Holding registers (analog, read/write)
#
# Function codes
# 01 = read coils
# 02 = read discrete inputs
# 03 = read holding registers
# 04 = read input registers
# 05 = write single coil
# 06 = write single holding register
# 15 = write multiple coils
# 16 = write multiple holding registers

# Polling loop (do not poll faster than 1 Hz on shared bus)
while True:
    regs = c.read_input_registers(0, 10, slave=1).registers
    voltage = regs[0] / 10.0   # 0.1 V resolution
    current = regs[1] / 100.0  # 0.01 A resolution
    print(f"{voltage} V, {current} A")
    time.sleep(5)

Tip: the vendor’s Modbus register map is gold. Search for “<vendor> <model> modbus register map”.

1. Wire first. A $5 Cat6 cable beats every wireless protocol. Run conduit if it’s a permanent install.
2. Pick the boring protocol. If Wi-Fi reaches, use Wi-Fi. If RS-485 works, use RS-485. Don’t add a LoRa gateway because it’s more interesting.
3. Don’t run a cloud dependency if you don’t have to. A local MQTT broker + Home Assistant beats Tuya + Alexa for a co-op who wants to learn.
4. Avoid vendor-locked ecosystems (Zigbee-only hubs that don’t bridge to MQTT, Tuya-only cloud, proprietary 433 MHz). Open standards survive longer.
5. Avoid mixing too many protocols. Three radios in one sensor is more failure modes. Pick the right one.