Next up we will be looking at TCP, the Transmission Control Protocol.

TCP is a procotol far more complicated than UDP, as in addition to ports, introduces a handshaking process, a notion of a “connection”, message acknowledgement for reliable transport, flow control, timeouts and a lot of other aspects.

Luckily these aspects are mostly abstracted away by the software we use and we can treat a TCP connection as a reliable pipe where data put in on one side will make it to the other side eventually.

NodeMCU Server

The code snipped below starts a TCP server on the microcontroller board that waits for connections.

clients= {}

function socket_to_cid(socket)
   local client_id= nil

   -- Go through the list of sockets and
   -- find the one matching the requested socket
   for cid, client in ipairs(clients) do
      if client == socket then
         client_id = cid

   return client_id

function broadcast(message)
   for cid, client in ipairs(clients) do


function on_tcp_data(socket, data)
   local stripped= data:gsub("\n", "")

   local client_id= socket_to_cid(socket)
   broadcast("Client "..client_id..": "..stripped)

function on_tcp_disconnect(socket, code)
   local client_id= socket_to_cid(socket)

   table.remove(clients, client_id)

   broadcast("Client "..client_id.." disconnected")

function on_tcp_connect(socket)
   socket:on("receive", on_tcp_data)
   socket:on("disconnection", on_tcp_disconnect)

   table.insert(clients, socket)

   local client_id= socket_to_cid(socket)
   broadcast("Client "..client_id.." connected")

function setup_tcp()
   local server = net.createServer()

   server:listen(5000, on_tcp_connect)

   print("listening on port 5000")


Replace the content of you application.lua with the code above, upload it, start picocom and reset the microcontroller.

Netcat client

You can now open a new terminal and connect to the server using netcat (note the missing -u switch compared to UDP).

[user@computer ~]$ nc 5000

Task: Using multiple terminals and netcat sessions and the sourcecode find out what the program does.

Task: Edit the program so that the first client can turn on the LED when she sends the “on” command and the second client can turn off the LED when she sends the “off” command.