Teensy Networking Tool

There are obviously proper tools for inspecting networking gear, but they are prohibitively expensive, especially considering our primary use case. We often wish to discover the switch port to which a given device is connected without having to chase tone. Our current solution is to use Wireshark and watch for LLDP frames matching our equipment's OUI. This works great, since our gear reports hostnames, port IDs, management addresses, and even firmware version. Even better would be a pocket-sized standalone device capable of sniffing LLDP. That is exactly what we intend to build with this project.

LLDP frames are of ethertype 0x88cc. Their payload consists of three mandatory TLVs (type-length-value structures) and a variable number of optional TLVs. Each TLV has a 7-bit type field, a 9-bit size field, and a variable-length value field. TLV types are given in the below table.

For this project, we are primarily interested in TLV types 4 and 5.

ssilverman/QNEthernet is a networking library specifically designed for the Teensy 4.1. Though most of the library is geared toward building client-server applications, it does include a class for directly reading and writing Ethernet frames (appropriately called EthernetFrame). This class is derived from the Arduino Stream class and functions similarly to the EthernetUDP class. Reception is invoked by calling parseFrame() to read the next frame and data() to return a pointer to the frame data. parseFrame() returns the byte length of the frame, or -1 if no frame is available.

The Teensy 4.1 is a highly capable microcontroller the size of a stick of gum. We selected it for this application for its integrated DP83825I Ethernet PHY.

We are currently undecided as to how or if we should implement OUI filtering. Perhaps the code could check for a config file on the microSD card in setup()? How should such a file be formatted for efficient parsing? How should we store the filter list internally¹⁾?

loop:
  check link status
  if down:
    indicate link down on display
  else:
    wait for new frame
    if ethertype is 0x88cc:
      read TLVs of interest
      output values to display
    else:
      discard frame

Parsing TLVs should present an interesting challenge thanks to the way their data is packed. We could interpret them as follows:

struct tlv {
  byte type;       // always truncated to 7b
  short length;    // always truncated to 9b
  byte *value;     // pointer to the first actual data byte
}