Eddy Zhou

Networking Primer for Robotics and Beyond

4 min read

What is Networking?

At its simplest, networking is how two or more devices exchange data. Every network is built on three questions:

  1. Who am I? (identity → IP/MAC address)
  2. Who are you? (discovery → ARP, DNS, broadcast)
  3. How do we talk? (protocols → Ethernet, TCP/UDP, HTTP, ROS2 DDS, etc.)

Networking can happen:

  • Directly, over a cable (Ethernet, USB-C networking).
  • Through infrastructure (routers, switches, Wi-Fi).
  • Or virtually (VPNs, bridges, Docker networks).

Core Concepts

Layers (OSI Model)

  • Layer 1 – Physical: Cables, fiber, Wi-Fi radio.
  • Layer 2 – Data Link: MAC addresses, switches.
  • Layer 3 – Network: IP addresses, routers, subnets.
  • Layer 4 – Transport: TCP/UDP ports, reliability vs speed.
  • Layer 7 – Application: ROS2 DDS, HTTP, SSH, NTP, etc.

When debugging, always ask: which layer is broken?

Addresses

  • MAC address: Unique hardware identifier (e.g., 00:B0:9D:1A:F0:0E). Works at Layer 2.
  • IP address: Logical identity (e.g., 10.8.0.18). Works at Layer 3.
  • Port: Application endpoint (e.g., TCP 22 = SSH). Works at Layer 4.

Subnets

  • Networks are divided into subnets (ranges of IPs).
  • Defined by an address and a mask:
    • 10.8.0.1/16 → covers 10.8.0.0 to 10.8.255.255.
    • 192.168.1.1/24 → covers 192.168.1.0 to 192.168.1.255. Devices can only talk directly if they are on the same subnet.

DHCP vs Static IP

  • Static IP: Manually assigned. Predictable but brittle if reused.
  • DHCP: Dynamic Host Configuration Protocol. Device asks for an IP, server assigns one. Often paired with static leases (MAC → fixed IP).
  • Link-Local (169.254.x.x): Fallback if DHCP fails and no static IP is set.

Routing and Gateways

  • If traffic is destined for another subnet, it is sent to a gateway (a router).
  • Example: Robot sensors (10.8.0.x) talk locally to the robot PC, which routes internet traffic out via a second NIC.

DNS

  • Human-friendly names (like google.com) → IP addresses.
  • Optional for sensors (often direct IP is fine).

Typical Networking Patterns in Robotics

  1. Isolated Sensor Network (I prefer this, as it’s the only one I know how to do :P)
    • Robot PC acts as DHCP server.
    • All sensors are on a private subnet (10.x.x.x).
    • No internet exposure.
  2. Dual-Homed PC
    • One NIC faces the internet (enp6s0).
    • Another NIC faces the sensor network (enp7s0f1).
    • Allows development SSH + stable sensor comms.
  3. Bridged/Switched Network
    • Multiple PCs/sensors on one subnet via a switch.
    • DHCP server assigns addresses.
  4. NAT/Forwarding
    • Robot PC routes sensor network traffic to the internet (optional).

Common Tools

Linux Commands

  • ip a → show addresses.
  • ip link → show NICs.
  • ip neigh → ARP table (who’s connected).
  • ping <IP> → test reachability.
  • traceroute <IP> → path to destination.
  • tcpdump -i <iface> → sniff packets.
  • dig <hostname> → test DNS resolution.

Config Tools

  • Netplan → declarative config on Ubuntu servers.
  • NetworkManager (nmcli) → dynamic config on desktops.
  • dnsmasq → lightweight DHCP + DNS server.
  • iptables/nftables → firewall and NAT rules.

Debugging

  1. Physical layer: Is the cable plugged in? Is the link light on?
  2. IP layer: Do devices have IPs in the same subnet?
  3. DHCP: Did the device request an IP? (journalctl -u dnsmasq -f)
  4. ARP: Does ip neigh show the device’s MAC?
  5. Ping: Can you ping the sensor?
  6. Application: Can you open the driver (ROS2 node, SpinView, etc.)?

Common Fuckups

  • 169.254.x.x addresses: Device couldn’t reach DHCP → stuck in fallback.
  • Multiple managers: Netplan vs NetworkManager fighting for control.
  • Wrong MTU: GigE cameras often need mtu=9000.
  • Overlapping subnets: Internet NIC and sensor NIC must not be on the same subnet.
  • Firewall rules: Block discovery protocols (e.g., GigE Vision discovery via broadcast).

Practical Scenarios

  • GigE Camera bringup:
    • Disable Persistent IP.
    • Enable DHCP.
    • Ensure PC runs DHCP server (dnsmasq).
  • GPS/IMU bringup:
    • Default static IP may need to be changed.
    • Enable DHCP if supported.
  • ROS 2 multi-machine:
    • All machines must share the same subnet and domain ID.
    • Ensure multicast works (DDS relies on it).

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